Many people who are not used to an HST transmission try to use the HST pedal like a gas pedal. If they start to bog down they press the pedal harder, which is the exact opposite of what you want to do. Give it more throttle if needed, but let up on the HST pedal (in effect shifting to a "lower gear") or shift to a lower range (you can't really get a lot of work done in the highest of the 3 ranges - that's more of a "transport" gear.)
Also, if the hydrostatic transmission is getting worn out, motion will stall out easier.