Carl C. Please don't interpret my comments as meaning that you can't put a 20hp brake on a 30hp drive. You surely can.
You will be limited in braking capacity by the chopper rating. But, if 20hp of braking will stop your machine in an acceptable time, then you can use the existing hardware. But, it seems that that hardware isn't working at all.
To maximize the capacity of the 20hp brake, you must find the minimum acceptable resistance value for it and provide that resistance. As to the wattage, since you've got a 20hp brake, the maximum resistor wattage would be the 15kw I mentioned above. If the duty cycle is lower, you can get away with less wattage down to one-tenth of the above rating for an occasional stop with completely cold resistors and no repeat until the resistors are fully cooled again. From what you describe with the centrifuge, 12-15kw may be required.
Remember that the drive software must permit the DC bus to ride up high enough to activate the chopper. In the software somewhere is a parameter that alters the decel ramp to avoid a HV trip. This feature must be turned off for the chopper to work.
Otherwise, you've got hardware problems. If, on stopping, any voltage at all appears across the resistors, its not an internal fuse. I'd check the DC bus under braking conditions to be sure it rises above 780 volts. If it does and the chopper is still not putting out any voltage to the resistors, the chopper is bad.