I've been playing some Tabula Rasa because I want to make sure I see what there is to see before it shuts down next month.
When I'm trying out a new game, I try to let the game teach me what the gameplay is meant to be through its mechanics, rather than looking it up on a website or strategy guide. After playing awhile in TR, I've decided the gameplay the game is encouraging me to adopt is collecting many weapons of different damage types, and switching among them often depending on which enemy I'm fighting or the stage of the fight.
The reason I've come to this tentative conclusion is that the game introduces enemies that are immune to certain damage types at very early levels, and robots seem to die much faster if you hit them with EMP weapons. There also seem to be a different weapons that are advantageous at different times in a fight, such as when an enemy's shields are down. This makes sense to me from a game design standpoint, but the truth is I can't tell if I'm imagining it.
I've also had problems like this in other games, especially games that are in genres that are full of conventions, like shooters. In some games, accuracy is affected dramatically by crouching or firing in bursts. Some have dramatically different hitbox effects, such as headshots that kill instantly, or leg shots that slow people down. Whenever I play a new shooter, it takes me days to figure out which common shooter mechanics are included, and which aren't.
It's ok to just give players advice directly
It's important to constantly and subtly reinforce the player behavior that you want, but it really is ok to just come out and say it too. If my weapons in Tabula Rasa had notes on them mentioning that they were good at taking out a shield, or a plant-based lifeform, or if it was part of the tutorial, I could be sure that swapping weapons often during the fight is gameplay the designers intended me to pursue. Likewise, a special UI element or scrolling combat text notification for "correct" damage that hits an enemy would help players learn these things through trial and error.
Every designer designs their game with an ideal gameplay in mind that will be the most fun or effective, but if the player doesn't find out them quickly, it's likely they'll end up quitting in frustration. Don't make your players close the game to search for information on what they're supposed to be doing, because there's a chance they'll never open it back up again.