Hello,
From your question, I'm assuming that you don't want it to return to -20 dBm once it reaches 0 dBm. You might have already done this but here are some things you can check.
For the Configure Power Modifiers VI, make sure you have TRUE going onto 'step'. and 1 going into 'Step'
Also in the Configure RF Power Output VI, make sure your Entry Mode is Numeric(True).
Hope this helps.
Jack J.
Applications Engineer
National Instruments