I had this issue once. The screenshots were not actually being uploaded even though it looked as if it were. I think the solution was to do it a screenshot at a time instead of dragging and dropping a group of them. It was something tedious at least. Sorry I can't be more helpful but as I'm sure we all know, app store connect has a lot of issues.
Topic:
App Store Distribution & Marketing
SubTopic:
App Store Connect
Tags: