Feel silly asking this, but is there a trick to RPM mode on Malibu Speed Control

[MalibuWantToBe]

The speedometer paddle wheel broke off my 2008 VTX, I'm waiting for my Nautic Laugic GPS to come in to fix the speedometer/MPH setting on the Speed Control.  In the meantime I thought I could just use RPM mode.  I hit the red button to get it to RPM mode, I set the RPM's, in my case 3340 to emulate 30 MPH, then hit on and the screen shows HOLD.   I then hit the throttle and and it never seems to engage.  With MPH, it would beep and then show me the canyon underneath and where my throttle was in the canyon, telling me up or down if I the throttle was too low or high.   In RPM mode I don't get a beep or a canyon, or the boat holding the RPM's steady at all.

Is the RPM mode still dependent on the paddlewheel?  Is there another trick to setting it?  I assumed when it went from OFF to HOLD that it was set?

Any input would be appreciated.  Using an app on the phone to show MPH works okay, but this week has been glass on the lake and when I get done and ask what speed she was pulling me I get 'Oh anywhere between 28 and 32'.   I can feel her messing with the throttle in my cuts.