Michael, I have seen this before when using a board other than the HobbyCNC. I think what’s happening is that there’s not enough torque to get the motor & all that’s attached turning.
Make sure that everything is tight mechanically (bearings, belts not too loose/tight, etc) and also double check that your voltage regulation is set correctly on the HobbyCNC board (too little and you won’t get enough torque). You can see how to do this with a multimeter in their documentation.
You could also try lowering the acceleration, though 2i/s seems pretty conservative already.
When you buy any HobbyCNC product, you’re automatically eligible to join their Yahoo troubleshooting group. I’d start there, or maybe just write an email to Dave Rigotti at HobbyCNC. You’ll want to note the peak draw for your motors, the average draw, and multiply each by 3 (because sometimes all three are energized at once). Compare this to the datasheet on your power supply. I think voltage fluctuation isn’t as big a deal for steppers, but I don’t understand the reasons well enough to make specific recommendations.