OK, my bet is that you merely need to turn up the regulator some. You do that by turning in the set screw on the left side of the regulator (I believe, too many guns in my head to be certain). If that doesn't work, turn the set screw out. One way or the other will raise the pressure shown on the gauge. Turn it up to where you want, and then shoot a string of 5 shots off to rebalance the regulator, and readjust if it changed.
You should only need to up it a little bit, between 300psi and 400psi. Don't go over 400psi as the you start running the risk of blowing the valve orings and such.