Default FB Messenger sync problems, pls help

Hi guys,

The first weeks with my Moto G everything worked perfectly fine but now i have a problem and i will try to explain it.

When a friend on facebook sends me a chat message i will get a notification on my Moto G which got the Messenger app installed. Now before, when i would read the message on my PC via facebook.com, the notification would automatically disappear on my phone, but now this no longer happens! The notification stays, even after reading the message on my desktop. So i now have to wake up my phone, unlock it, start the Messenger app and then (and only then) the notification will disappear.

What could cause this problem?

Thanks in advance!!