I am about 1/2 through 3⭐ in the village and have had trouble attacking with mounting. Sometimes I use "X" and sometimes "A"? Is there a reason it jumps between the two, is there a way to know which button I need to mash before I mount?
Mounting has a lot to do with the monster. As long as the attack hits the monster, you get a chance to mount it.
However, the attack must knock it down in order for you to mount it. An air attack can connect and do damage, but if it doesn't knock it, you won't mount it, there's no need to mash any button.
If an attack knocks the monster but you don't get to mount it, you have to run and get as close as you can to it as fast as you can in order to mount it.
As for why some attacks will knock the monster and some don't, it depends on the monster. I've seen that it's way easier to knock them with an aerial attack when they're tired (I get a lot of opportunities to do it, as I use Insect Glaive), also, the green bar that you have to fill with be closer to the right the more tired a monster is.
Also, the more you mount a monster, the more difficult it will be to do it again.
While you're mounting and attacking a monster, you can break parts on their back if any.
Oh, and if you have Earplugs, you can hit the monster while they roar to try to get you down as if the monster head was green in the bar.