BLewis - finally cracked the code on this issue. I use Onyx PosterShop but should be kind of the same. I've had several jobs when I needed to use the full width of the material and could not, so this latest one was the charm.
a) Go to Settings on the printer (the wheel) - Substrate - Substrate Handling Options - Right Edge Position - CHANGE THE RIGHT EDGE POSITION TO 0
b) Watch the HP video
c) Substrate settings: has to be NM - no margin, which I have been using for a while but not being able to print with that.
d) I told the RIP and also the printer (also on the substrate settings) that the material was wider than it actually was, so the material is 30" but I told the RIP 32" inch roll:NM
e) Justification to the LEFT EDGE ( I tried the right edge and it printed way out of the substrate and not the right width of the print either.
Can't tell you how excited I am about this, wish I had figured it out a few days ago, as I had 3 60"x120" prints on 60" material. Better late than never.