Some of PDFs cannot be synced to IOS device.

I've been using Mendeley IOS 2.1.2 for a while and it works perfectly for me except one problem: some of PDFs I saved on mendeley desktop cannot be synced to my ipad air. Beside these titles on mendeley IOS, there is only an empty box, instead of a bookmark box indicating that the item is downloadable. It looks very puzzling as other PDFs are downloadable. It looks like mendeley IOS just "forgets" these files. I have tried to re-install mendeley on IOS and enabled the sync option in edit settings, but it doesn't solve the problem. Thanks very much.