First thing you have to do is decide if you have an electrical, mechanical or software problem. Lets start with electrical. Here's some specs if you don't already have them http://www.kelinginc.net/KL-5042.pdf for your driver. Make sure step and direction jumpers are set up properly and motor current as well. You may only want to supply 80pecent or so current to your motors to prevent overheating. Performance won't be hurt much by that.
Now comes your power supply. This impacts your performance the most. The specs say 20-50v supply. Higher voltage allows for higher speeds. Closer to 50v the better but not too close. 36v is OK. Next is current, Specs didn't say what percent of motor current the drivers require for proper operation. Your supply is 6.6A and your three drivers supply 4.2A each for a total of 12.6A. Your supplying about 50% That might be your problem. Amperage controls the amount of torque you have especially at slow speeds.
Also turn down your acceleration in the software until you get things working and then increase it when you know everything else works fine.
What do you mean by when it works? Do your motors stall? Does the system not start at all? Might hold a clue as to your problem.