files in my private groups are not synced with mobile devices

The files I upload in my private groups are not synced with my tablet or my phone - samsung, Android version 5.0.2 both - No problem to sync those files between my desktop computers but none synced with tablet nor Phone (I know that private groups doesn't allow to share files with others but the problem is I couldn't share the files with myself in mobile devices, but the same files are synced and available with my 3 desktop computers)