Layers not visible after closing and re-opening saved file

Hi, I hope folks can help with this issue.

I have created and animation but every time I close and reopen the file some of the layers are no longer visible. They keyframing edits are still in the timeline, the layers are not hidden, and when clicking on a frame for one of these layers, it seems to be empty. I have re-edited a few times but the same issue seems to happen (with different layers) each time I save, close, then reopen later.

Does anyone know any solutions? I don't want to have to re-edit AGAIN or try to complete the animation without closing the file -Thanks!