Happens me, just make an action while looking at specific HP bar. e.g While you move, attack etc. check that HP bar. the damage that "does-not-seem-to-be-inflicted" will get, well, inflicted. No problem. It is just visual. a genjutsu!

Happy hunting!

I never used a drone in the whole game.

If the drone doesn't fire, you:
1. Don't have enough energy
2. Still overheat when did it twice
3. Don't have enough ammo/rockets
4. Don't do anything when the time ran out or it's too late to do your 2nd move

If non of those, you encountered a serious bug