Things to consider:
Are you wired up correctly?
I believe so, they work just peachy without any load, and they come prewired from the factory. All I did was extend the leads on them and add a connector to plug into my enclosure
What kind of amperage is your power supply? It looks like these things can handle up to 6amps.
20A max could I be pushing too much current thru them? Hope I didn't burn anything out!
Do you know your actual torque requirements for your system? A holding torque of 2 Nm (~282oz-in) really is about 70-80% of that in the very low RPM range, then will continue to decrease. Really you're probably at around 225-200oz-in torque in the 100RPM range.
I did not, but I use smaller steppers on my other mini mill, and they have WAY more torque at 48V than these buggers do.
What RPM are you running your steppers at? I've tried varying the RPM with no real difference.
I've gone down to 10RPM up to around 200 RPM with no noticable difference