Some guns, especially ones that have a tilting barrel (most pistols) can be finicky about adding a big weight at the end of the barrel. most surpressors nowadays have methods to sorta help counteract the weight, but the gun can still jam or not cycle if the ammo isn’t powerful enough.

in this case, it looks like they didn’t test out their setup well enough, and had to manually rack the slide to eject the spent case and chamber a new round.