Hi praveen
There are Lot of reasons which leads to this scene but the primary issue is that the Sync is failed. Check the logs that Performing simultaneous synchronizations is not supported. Please try again after the current synchronization is completed" means two sync operations were triggered at the same time.
or there might be any conflict with the offline database sync which occurs when the mobile has the cache of database structure and the new sync is trying to execute.
To verify this just try to clear the cache and check.
I hope this helps!!