User getting not informed in ACTIVITY when SharePoint library is updated

Hello

After a couple of weeks user are not getting informed in Teams ACTIVITY when a new or updated document in the library is created. It is a bit weird as some are getting updates others are not. But the ones not getting updates, getting updates from others.

If we are looking inside the settings of the client, everything is ok. They will get notifications in their ACTIVITY in Teams.

How can we fix that problem?

Thanks in advance.

Kind regards

Michael Damaschke