Make sure Motor Mix is selected as an input device in Pro Tools: MIDI menu > Input Devices. This may also be caused by a faulty MIDI cable or port. Make sure both the in and out MIDI ports are being employed, both on the Motor Mix end and the MIDI interface end. Try using different MIDI cables and switching to different ports on your MIDI interface. Ultimately it may be necessary to create a new OMS Studio Setup to regain normal communication.