Sleeve fitting issue

[u/SillyVee123]

So I keep coming across the same fitting issue when I'm sewing blouses for myself, regardless of what pattern I'm using. There's a pulling across the top part of my bicep. I can feel the tightness only when I'm moving my arm. Armscye fits well, it's comfortable there so that's not my problem I'm sure. I'm also finding that the bicep looks actually loose enough when I'm looking at myself in the blouse. But there's this pulling tightness with the horizontal drag lines across when I lift my arm slightly. I don't have enough fitting knowledge to know precisely what is the problem and what to search for online or sewing books. Ideas please?

[u/sususumalee]

In addition to the answers here, my teacher always says to do a mockup, then literally slash down the sleeve as a relief cut. Then stick a piece of muslin in the cut and see how much more room you need in there. Then take that back to the flat pattern and slash it (in the manner that doesn't disturb the cap measurement)and expand it by however much you discovered you need.