Are you saying that it does NOT reduce the voltage to your motor right now, as in you are not sure if it is working? If that is the case, how are you determining this? A fast acting non-RMS digital multimeter may not allow reading of the true RMS voltage going to the motor. You need to use a true RMS reading voltmeter or an old iron vane analog meter.
Other than that, if the device seems to be working properly but you are not seeing a reduced voltage, it is probably set too high (or the ramp time is too short). This would most likely be explained by what dpc posted above. At setup, someone may have discovered that reducing the torque cause the motor to stall, so they kept increasing initial torque until it no longer did. But in increasing it, they essentially removed the soft start effect altogether! That would be an example of someone not understanding the limitations of soft starters.
If what dpc said is the case, a VFD may be your only choice. By altering both the frequency AND the voltage at the same time, the motor can be slowly started, but with full torque from the outset.