RTV900 '06 - No Power to Wheels

Hi All,
New to the forum..and looking for help

We have a '06 RTV900.
One morning, started it up and no power to the wheels.

* We measure 0psi at the Check and High Pressure Relief
Valve (machine started in neutral)
* So no drive to the Assist or Stationary Motors.
* Hence we focus on the control of the Variable Swashplate (HST Pump)
* No problem with Regulator Valve Assembly
(Feedback Level, Control Level, Regulator Spool)
* Pressure by Charge Pump at the Charge Relief Valve is good
- reading 87psi
* Suspect the servo piston position is not changing, regardless of
Feedback Lever position, so the pump's variable swashplate remains
neutral - hence 0psi
* To verify this, we adjusted the Piston Adjusting Screw to force the
piston to move. Sure enough, we get power

Why would this need to be adjusted?
Could it indicate issues e.g. piston seals

Just not sure how to interpret the results of our "debugging"!
Any advice is greatly appreciated.

RTV900 '06 - No Power to Wheels