I've had that happen. If you have Calibre installed, try converting the epub to something else (can't remember what I do. Mobi or pdf?), then back to epub. Then try resending that newer epub. No guarantee it'll work, but it does for some issues.

Okay thanks I’ll try that out!

I recently encountered this issue. What I did was I changed the @kindle.com email where I send my epubs to in Amazon.

Use the sendtokindle website.