Players keep falling

Im having a problem keeping the defensive player that i manually control from falling when Im trying to blitz with him. What am i doing wrong?

You are either hit sticking instead of waiting for him to engage to do a move... OR you are using finesse moves with a guy who isn't a finesse lineman.

Oh so i should wait for the player to engage & then take over

Well, who are you.. a lineman I'm assuming? For example.. a bull rusher like Jamaal William, if you try to spin.. he'll fall. Or if you hit an up-stick/down-stick move too early, the guy will hit stick instead. LB's can usually do power and finesse moves.. not necessarily well, but they won't fall.