Thumbnails broken

Hi All

I'm new here, I have a SP list that has an image column, it's an older list so the images are still stored in Site assets. It's the back end of a Powerapp. The problem I have is that some of the images won't render in some thumbnail sizes in my app. So some images render small, medium and large but not full and others render in small medium, full. I think it has something to do with when I updated a record in my app. but for now I would like to know how to repair the image column. I'm fairly sure it's Sharepoint issue, as when I select an image in the list, it sometimes does not preview as a large image.

Any help would be much appreciated.