Keep missing chat

Keep missing chat

My desktop and mobile, both are configured to receive any online visitor who has initiated a chat. This was working perfectly. However since last 10 days, I get a notification of miss chat ONLY and do not get a notification when a visitor initiates a chat.
Any idea if this is a common problem OR only I am facing. In case only I am facing, is there any ready solution?

Thanks in advance

anirban