If MIX messages aren't showing up for you, please check these areas in the MIX Messages window first.

Also, if you have not been clearing out your Messages by marking them as Viewed, then you might have exceeded your Max #. The Max # is editable by staff. However, you should clear out your existing messages, by clicking on each message and then clicking the Viewed button.
In this example, changing from 12 to 250, makes previously unseen messages pull in.