No push notifications for facebook messenger

After hard reset my Nokia Lumia 620(windows phone 8.1 preview for developers) there is no push notifications for facebook messenger when battery saver is on. Before the reset there were an option for allow app for run on background tasks and allow app even battery saver mode on..??

Try checking whether battery saver is aggressively saving the battery and check also on the usage of the app if it is allowed

But what to do to get an option for allow background tasks??