I bought SleepCloud premium, but the app doesn’t sync

The following should hopefully solve your problem: start sleep as android wait for ~15 minutes click on menu > backup > upload to cloud. The upload should be unrestricted at that point. Let us know if that does not help.

Why does backup always fail for me?

Please first check to you have sufficient connectivity, ideally a home WiFi at the time of synchronization. If synchronization is still an issue, please send us a debug report directly from the backup add-on in Settings > Backup > Menu > Send debug.