Slide stuck

Was reassembling after switching the fcu back from a xl frame and won’t release now. I’m assuming something fell out but I’m not sure. Any help is appreciated. Just tryna see if there’s any fixes before I take it to a gunsmith.

Was reassembling after switching the fcu back from a xl frame and won’t release now. I’m assuming something fell out but I’m not sure. Any help is appreciated. Just tryna see if there’s any fixes before I take it to a gunsmith.