HST and loader are completely separate systems. They only share the hydraulic fluid.
HST malfunctions can be in the linkage. With engine off, look close at how the linkage under the floor moves while someone rocks the pedal back and forth. There are several rods, pivots, levers - look for anything loose, bent broken, blocked etc. Hopefully its something simple on the outside.
Please say more about how this symptom came on? Smooth road or driving over brush etc? Ever act up before? Does it move forward in all 3 gear ranges? Same from cold start or does it get worse as fluid warms up?
For more reading on a similar issue look
here.