itunes could not backup this ipad bexause a session could not start on this ipad

"itunes could not bacmkup the ipad (Omar's iPad) because a session could not start on this ipad" error shows to me every time i sync my ipad 3 or when i try to backup it, but in the meanwhile it syncs and backups my iphone 4s and my sister's iphone 4😢😠 plz can any one help me???