Okay old timers!

I need your experience!
I modified the css of an epub then side-loaded it back onto my NOOK Simple Touch. Now the book will open to the cover, but as soon as I attempt to turn to another page the book closes, with the reader returning to a view of the bookshelf. The same book works just fine on the PC in calibre's eBook Viewer.
Has anyone had a problem like this before? Is it possible that there's something in the book that the NOOK can't handle? Does anybody know what's going on?
Thanks in advance for the help!