If users are getting other notifications but not these, you need to check that the Confluence Mentions Plugin is enabled. The instructions for doing this are here.
If it is enabled:
Is this all users, or just some users?
Does it happen with individual names or only when you mention a group?
In that case the one thing I can suggest is that you ask one of your affected users to reset their avatar to the default, as a corrupted avatar can cause this problem (see here for more information). If that works then they should be able to reapply a photo to their avatar without the issue returning.
If that doesn't work then I'm out of ideas so if no-one else suggests anything then you'll need to put a support ticket in and ask Atlassian to investigate.
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