Every designer has experienced it: that moment during playtesting when players just don't "get it" and the designer has to change something. It's always a point of frustration, and if it happens enough times, the designer might even harbor some resentment towards the very players he's trying to please; it's only natural.
Once a designer starts to care less about the player, it will show in his work. When the design work suffers, the player suffers, and in turn the designer suffers even further, a vicious cycle indeed.
Another problem can arise from self doubt, why aren't they getting it? What am I doing so wrong? It's easy to feel the control slipping away, and soon everything in the game becomes suspect. Things that didn't need changing are altered, and productivity goes into a tailspin.
"I don't get it" can encompass a multitude of problems from getting lost, not knowing what to do next, not understanding how a certain gameplay mechanic works, or just totally missing the point about what the parameters of the game are.
Grappling with these frustrations just comes with the territory, fortunately there's a bounty of methods to cope with these problems at your disposal. Let's take a look at some of the various balms and salves that can ease the pain. As with all medicines, there's side effects, and their description doesn't touch on instances of ineffectiveness or inappropriate use.
These are meant to just offer some possible fixes to problems, they are in no way meant to solve underlying issues, but just a collection of band-aids, field dressings, and tonics to help give a designer some relief in a pinch.
Lead by example: If the player is set in a tropical paradise but never seems to understand that falling coconuts are dangerous until it's too late, they might need an observable example. If the player is able to see and hear insects getting squashed by falling coconuts, the danger is now soliciting itself.
Show the end result: If the player needs to use the flame sword to melt ice blocks but isn't making the connection, show an ice block that's already melted. The player now knows that the ice blocks can be destroyed, and it's apparent that they can be melted.
Mnemonic devices: If the last time the player had to look for a hidden passageway was in the graveyard, but now it never occurs to him to look for a secret route while in the factory, find an excuse to place a tombstone in the factory. The player can't help but remember what happened in the graveyard, and there's now a tangible connection between the two places. It's totally out of place, but that's the idea.
The take away: If 3 hit combos yield more gold from slain enemies than regular attacks, but the player seems to keep forgetting that fact, start stealing his money in some way he's aware of. He'll get defensive about his funds, become more conscious about gold in general, and may remember that 3 hit combos give him more gold because he's been robbed and is eager to make up the difference.
Guarding the goods: If the player needs to have the raft to get to a vital area, but looks out at the dock and is clueless that the raft is nearby, elevate the importance of finding the raft by enshrining it in difficulty. If the player sees a definite obstacle, he'll better understand that there's something of value once the obstacle is completed rather than just walking by something that appears pedestrian.
Put them in the proximity: If the player needs to talk to a certain villager to trigger the next quest, but he never takes the initiative to talk to the wise old sage, find reasons for him to keep returning to the village. "This weapon you found isn't terribly useful but it's very valuable" may spur him to return to the village to sell it which gives him another chance to start up a dialogue with the people he bumps into on the way.
Polarization: If the player should get in the habit of putting on his goggles to see better in the mist, but thinks that low visibility is just a fact of life, make a better distinction between "you can go here, but not here" by making the mist extremely thick in some places, and less dense in others instead of one homogonous value of visibility. Once he's been prodded to put them on to navigate the thick areas, he'll be able to observe that he sees better even in the less dense mist, and now will always put them on when mist is encountered. No further polarization is needed, the habit has been established.
Employ what you fear: If the player can't make the connection that he needs to sneak up on armed guards instead of taking them head on, see if there's a way to make the guards sneaky themselves. When the player becomes paranoid about clandestine guards, he'll be less motivated to give up his position and may start becoming more discreet in how he dispatches his enemies.
It happened to me: If the player doesn't understand that robots are weak against water, give the player a robotic trinket that he can't hold on to because when he crosses the river it short circuits. He may be lamenting his loss, but he knows why he lost it, and how his problem applies to other aspects of the game.
Crank up the volume: If the player needs to duck to avoid the occasional laser beam attack but often fails to do so, create a situation that is brimming with laser beams rather than the planned handful. When confronted with so many lasers, dying from laser beams is no longer is an annoyance, it becomes a major priority and gives them a better chance to hone their skill and be less forgetful.
Guilt by association: If a certain type of thorny monster can only be killed with grenades, but players don't try everything and conclude he's invincible, surround the thorny monster with enemies that can die in a less specific way. Once the player sees that the thorny monster is all that remains, he may rethink his stance and try experimenting with different tactics.
Killer Combos: None of these fixes are mutually exclusive; sometimes you can show an example that also acts as a mnemonic device, or place them in the proximity while showing them an end result. Doubling up on fixes can often turn a negative into a positive; if something's so out of place and odd, it draws attention to itself, which may help shift the player's attention to an issue in memorable and entertaining way.
It goes without saying that its extremely easy to poke holes in these fixes, they are far from a replacement for sound overarching design. While a game composed mainly of quick fixes would hardly constitute an enjoyable one, let us not snub our noses at the humble band-aid, it's there to fix minor problems and is very effective in that capacity.