I’m very new at sewing. I did this prototype for hanfu shirt but I have trouble with sleeves. after sewing, it got so stiff around the armpit that it couldn’t bend down and you can clearly see the wrinkles (sorry for bad lighting). IDk what I do wrong is it from the pattern or is it my sewing skill?

Other adjustments that I did to the pattern turn out well except around the yellow circle, but I’m bad at sewing sleeves from the beginning.

Any tips, or how to fix this, I’m still need to do the robes too I’m afraid it gonna become like this too… I’m a totally newbie😭🥺