Why do i need to keep ticking books to sync

When syncing my iPhone and iPad from iMac iTunes, I use the "selected books" option. Sometimes certain books will un-tick themselves and the next time I sync, I need to re-tick the selection. I know they are un-ticking themselves, as I have certain favourite authors which I keep synced at all times and sometimes I can see that certain books of theirs are un-ticked. Can you please tell me how to fix this issue?