I have a similar if not the same problem.
My transmit power is very low in all modes.? This is with a 1:1 SWR.
But, if during transmit, I slide the volume slider up I can raise the power to what appears to be full power.??
On the next transmit cycle it reverts back to low power again.
I could maintain the higher power out if I just slide the volume slider back up during each and every transmission.
I'm talking about the little blue speaker icon in the upper right hand corner.? The one that would normally control the speaker volume out on the PI.
AE9J