Reasons can vary, as others said.
I can't sync any longer due to this:
https://www.mobileread.com/forums/sh...d.php?t=352381 (or better, I can sync only if I delete the collections first). I don't know if it's your case, but if you have lots of collections or lots of books in a single collection, your case may be the same.
Another reason is a corruption of the DB (in that case, logging out and logging in again is the only solution you have). I suggest you to use Calibre to back up reading status, highlights/notes, and collections, so you can recover it all after logging out.
Sometimes repairing your account on the device does the trick. Or deleting your wifi connection and reconnecting again (or connecting to another net, if available).