General help

In Moodle 1.7+, when I logon, appears a pop-up window howing all new messages sent to me in bold (this is normal).
But, from some users, whem I click the message I cannot see the message (I have admin rights), but they are present when I go to "message history".
But the problem is not only this. When I logon the next time, I can see the users which sent the messages in bold again, an the messages keep not appearing.
So, whenever these users send me a message, I get an increment in the number of new messages from that user, but cannot read any, only if I go to message history.
Even if they do not send me new messages, the old ones keep being stated as new.

How to fix this?