I am working on a 3 axis build using vexta pk266-02b unipolar stepper motors rated @ 3.6v. I noticed in the manufacturers specs that they used 24v for testing? Why would a motor rated @ 3.6v need to be supplied 24v to reach its optimum potential?

During driver testing I have been using a 12v supply regulated down to around 6v which gives me about 4v @ the motor coils after the voltage drop when energized. The motor draws the spec 2 amps @ this voltage. The motor functions ok until I put some drag on the shaft then it bogs down fairly easily (kcam set @ 3200 steps/inch), however the voltage does not drop significantly under this load. Suggestions will be appreciated.