Are the wrinkles vertical or horizontal?

This is a page from Sew News Magazine, online their library often has articles from past issues, this magazine is great for clothing and learning how tos
the article I have linked is for fitting armholes

Here is a link that might be useful: sewnews mag. armhole fitting

A few years back--OK, 3 decades!--I took a pattern fitting class at my local community college. I think that we were told that wrinkles tend to point to the problem causing them, and it sounds like they are pointing to the bottom of the armhole. Use the link from rene54 for reference and stitch a seam that tapers from the notches on the sleeve to a maximum of about 1/2 inch below the original seam at the underarm--this makes the armhole bigger. We were also told to trim that section to 1/4 to 3/8 inch and clip the curve. Stitching a second seam close to the original prevents ripping.

One other alteration that helped a number of us in the class was to rotate each sleeve about 1/2 inch toward the front, but I think that was for wrinkles in a more fitted sleeve.

Sharon C.