FH slice return - it's the same as a FH volley. OP I suspect your FH volley maybe flawed... try this visual.
imagine there is a big piece of cardboard on your right side facing the target. you are gonna put your upper arm, elbow, forearm, racket shaft, racket face... everything on that cardboard..... now this is a very heavy unit. and you are gonna chip/block that serve back with this entire unit.
I believe this will fix your FH return and FH volley