1. Inspiration. Already mentioned. This is a great mechanic, but it's almost always forgotten. In addition, when it has been granted to the players, they don't use it because it's so valuable (?!?).
In part. The player having no concept of when they may get more is also relevant. If the GM has been forgetting inspiration, they get out of the habit of giving it out. Which tends to lead players to hoard it. Which means the GM doesn't
see it used, and so they forget it and don't give it out. Vicious cycle.
Another variant that I have seen vastly increases Inspiration economy flow - use Inspiration as a pool. The Pool maxes out at one point per player. If anyone earns inspiration, it goes into the pool. Any player may use it with agreement from the other players. You have a
stack of tokens on the table representing the pool, which the GM can see and track as a reminder of how much the party has on hand, and when they may want to offer more. Also largely eliminates the "You just did something cool, and I'd give you inspiration, but you already have one point, so you don't get any more."
With a pool, the party is actually incentivized to use at least one point every tie the pool is full, so they can accept new points if the GM feels they ought to have it. Raising the cap from one point per person might have similar effects - say your maximum inspiration pool is 1 point per character level.
2. Ad/Disad. What? Yeah. Other than specified instances (such as a spell or ability), players never seem to look for, or ask for, ways to get ad or avoid disad creatively. Which is a shame.
Has the GM regularly
demonstrated (not just mentioned in passing in Session 0, but actively engaged in) handing out advantage for clever play? If not, don't blame the players for not spontaneously engagign in, "Mother, may I?" play. Players will tend to reach for reliable solutions first - that means the rules that everyone has agreed apply. Expecxting them to come to the GM is... kind of making them plead for it, which isn't a great dynamic sometimes.