Synchronization via syncadapter does not work, what could be the problem?

Question

Android

Good afternoon!
I use Syncadapter to periodically synchronize data with the server. I noticed that synchronization starts only while the application is running (active or minimized). As soon as the application is closed, synchronization stops. What could be the problem?
I'm testing on version 5.1, the application has permissions.
When you click "Sync now" in the account settings - nothing happens.
If you place the application widget on the desktop, synchronization is performed.