Half the voices usually mean that you have forgotten to set your REV2 to be "Local Off", and at the same time are echoing the MIDI data your DAW is recieving from the REV2, back to the REV2... then what happens is that it plays one note when you hit a key on REV2, but also send that note to your DAW that then sends it back to your REV2 via MIDI, and play that same note once again... every note you play is played twice on top of each other... thus half the polyphony... if you set your REV2 to be "Local Off" (you can find that parameter in the global settings) you disable the keys on REV2 from the engine... thus it will only play the note it recieves back via MIDI... problem solved.
If Slave mode is set, then your REV2 will not work with either ARP or Sequencer or any tempo sync function, unless it also recieve MIDI Clock messages on the MIDI input... you DAW is supposed to send that clock, so enter your DAW's MIDI settings, and make sure it is sending MIDI clock.