SL/PE10 shoulder pain

I get the pain doing l chins as well. I'm not able to keep my serratus engaged and it feels like my shoulder blade collar bone and humerus are all getting smashed together while I'm hanging doing the exercise. Wall slides are helping a little and I am going to start doing db pull overs. It feels like an activation issue...how do I turn my serratus on!? Horizontal pushing/protraction exercises don't help and are painless. I need help with keeping my serratus engaged while pulling vertically specifically when I'm in an L or pike or tuck. Please let me know if u have any exercise suggestions. Thanks!