You could have blocked the user by mistake.  Normally if you find a user that you don't want to read what they post you press the little speaker icon next to the yellow info icon.  then you will see that you have this user ignored.  If you see the Heads up you can click on the unignore this user and you will then see their messages.  

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13