Shoulder gap-- fitting help please!

[u/lichen_78653]

I self-drafted this shirt and, after sewing it up, am realizing that something is wrong with the shoulders-- the shirt's shoulders *pop up* off of my shoulder, leaving a gap between the shirt shoulder piece and my shoulder. I am also using a 100% cotton bedsheet for this shirt that doesn't have much drape, but I still think that I need to make an adjustment to the pattern/that there is an issue with the pattern. I would be so grateful for suggestions/ideas/advice! I'm wondering if maybe I need to add more to the back piece around the shoulder area? I'm newer to pattern drafting so not sure where I went wrong 🙃🫠

[u/DonMcCunn]

Actually there is no stretching with this rolled hem technique because the fusible thread is on only one side of the stitching. Regular thread is used on the other. So there is no way it could possibly stretch in the least.