every 4th/5th shot is not engaging

So I just tried out shooting my aap because I built in some new internals because I wanna run it HPA. When firing with no bbs inside my HPA adapter everything worked flawlessly so I tried shooting some bbs and noticed that every 5th shot or so the trigger does not engage and the bolt gets stuck on the way of its return (yes I lubed everything properly). I‘ve come to notice that the point where the issue occurs could be the auto sear but I‘m not quite sure. Please notice that the stainless steel auto sear came with the hammer set I built in, so I upgraded that aswell (the springs are still the stock ones), I‘m also not able to push it down because the resistance is just so strong but I don’t know why since there is a stock spring underneath. As soon as I reinstall the lower in the upper the trigger engages again and the auto sear seems to be pushed down…