I've been dealing with a hand injury (right) that developed after a few months of consistent hard boulder for a few months now, and during my time off to rest (more or less 6 full weeks) I developed significant instability and a grinding sensation in my right scapula. My scapula seems to be able to move forward (toward the front of my body) past its normal range and general feels highly unstable. I don't experience much lack of strength while climbing (although I'm still taking it easy on my hand) however it does seem like like the muscles surrounding my scapula are tiring out a bit quicker than usual. If I move my scapula forward and shrug my shoulder, the grinding sensation can only be described as severe.
For a couple of weeks I did a bunch of exercises for my serratus anterior with no result and now have begun thi -regimen- physio-pedia.com/Scapular_D…;
Been doing it for a week with no improvement so far. It's just as bad as it ever was. It seems like muscle instability would be the obvious culprit, but none of the recommendations for that seem to be working.
Lower trap and as you mentioned, serratus, are very important to proper scap mobility/stabilization. Look up eric cressey's youtube (baseball PT/trainer) for more ideas and information on scaps - dude knows his stuff and has a TON of information on the scap and shoulder stuff. Good luck.
I have had similar issues for a while. More than anything swimming a few times a week with great focus on making sure my scapula is in the right position has made a huge difference. Years of shoulder instability alleviated in a month. Couldn’t recommend it more.