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Sounds like a Serratus Anterior issue to me.. could be a little of the Teres group.
I think there are some videos on here about trying to separate lat and serratus. I would try taking the pointy end of the Gemini and digging that in just anterior to the lat muscle belly. With the gemini wound up in there start moving your arm to that over-head squat position. Work that same motion with the Gemini starting way up in your armpit and move down until you run out of lat to put that thing in. Then I’d take a lacrosse ball to your serratus and intercostals.
Shoulders can be a pain in the ass. That’s what I would start with though.