I had experienced the same problem and I posted it in here, never really got a real response. But everytime this happened to me, it was after I booted or rebooted my MAC. Everytime I did that and opened up Protools, it would recognize the Mbox2, I could hear my mic through the mbox2, and the track was armed etc.....But protools was not receiving the signal and it was not triggering the meter in the mixer or edit windows.
What I still do, because this still occurs, is after I open up Protools I unplug my Mbox2 and replug it in. After that, it works with no issues. I figured my problem was MAC and timemachine related, but my Time machine is turned off and I still have this issue. So anyting anyone can suggest will be appreciated, until then I will keep utilizing my unplug method.......