Probably the classic beginner cleaning mistake when using ascenders on a big wall is not pay attention to what’s above you, jam your top ascender up tight against a carabiner or knot . . . and get hopelessly stuck. Your ascender needs to move a tiny bit forward for the cam to disengage, and if you slam it up hard against a biner with the rope stretched tight under your body weight, this is not gonna happen. (This becomes even more of a problem if the rope is heading off on a bit of a traverse. I learned this the hard way on my first big wall.)
A good rule of thumb: always stop your ascender about the width of a fist, or say 4 or so inches, away from a carabiner or knot. This will ensure you always have enough room to back it down or remove it.
Here’s the WRONG way. The ascender is jammed up tight against the biner, making it near impossible to move it up, down, or remove it from the rope.
(Note - climber, harness, daisy chain etc. not shown for clarity.)
Here’s the CORRECT way. The ascender is stopped about 3-4 inches below the biner. This gives you room to move the ascender up, down, or remove it from the rope and reattach it above the biner you need to clean, a common cleaning tactic on traverses like this.