I find that often i can't hit mobs they're standing right in front of me but no matter how many times i try can't hit them untill either another mob comes along or they move a little.

had something similar due to the evilcraft mod that adds ghost after you kill something. maybe it's the same thing?

Yeah that's what it is ugh really dumb was making a mob farm for xp but couldn't kill the mobs.

Caused by Evilcraft's Vengance spirit most likely, i dont believe it can be disabled as it seems to be part of the mod's main content