1. Run a line of stay stitching along the area of the armhole where the problem is. As you sew ,place your index finger directly behind the presserfoot of your machine so it causes the fabric to bunch up at the back. This is sometimes called stitch crowding. The tighter you push the more the seam will pucker. Don't worry,these puckers are good puckers.The idea here is to shorten the armhole so it will lay closer to the body.
2.On the garment side of the seam,press out the puckers so they won't show on the finished garment. Don't iron out the puckers in the seam allowanse. They won't show anyway.
3.To strengthen this stay stitching, take a piece of twill tape (I often use the selvedge of a soft fabric like silk)and on the wrong side ,sew it over the seam.
Finish the armhole as usual. On this dress,I sewed the lining and dress together at the neck and arms, then I sewed the facings to the outside of the dress, understitched and sewed each facing to the lining by hand.
The dress now fits well at the armhole and there is no gaping! A simple solution for a very irritating problem!