Help me with Viber

Hello!

Recently there was a problem. If my Viber is not open, then messages do not come. When the phone is locked or I use other applications, messages do not come. The sender has one tick instead of two, as if the message was not delivered. It's not about the lack of notifications, it's about the lack of message delivery if the phone is in sleep mode or I use other applications. I uninstalled and reinstalled Viber, checked that all notifications are active, but nothing helps. Help!