spindle override will not override RPM over the programmed RPM it will go under but not above is this a parameter that can be changed?
Are you running at top ROM now? The over ride will not go above the top ratedRPM for the spindle. If this is not the case, it maybe that the knob on the pot is not properly installed in the center of the pot sweep. It could also be a faulty pot.
These are the most usual things that I see for this situation.