It certainly sounds like a hardware problem on the board. The first thing to try is to change the potentiometer setting for the motor current going to the motor. Perhaps the potentiometer is actually in the right place, but the contact is weak and needs to be refreshed by turning it back and forth just a bit. Or perhaps you bumped it and set it down low unintentionally so it needs to be raised.
If this doesn't fix it, and you are at all handy with electronics, you may be able to do some troubleshooting.
Stepper motors have two independent coils. To explain the action, if one were using 1X steps, then one might see full current going to Coil A in the positive direction, followed by full current going to Coil B in the positive direction, then A in the negative direction, then B in the negative direction, then back to A in the positive direction and so on. (To get rotation in the opposite direction, it's like A+, B-, A-, B+, A+). The Printrboard uses stepper driver chips that actually modulate the transfers from current in A to current in B according to 16 "microsteps". The stepper chip receives "enable", "direction", and "step" commands from the Atmel - but those signals seem to be OK, or you wouldn't be getting any life at all from the motor.
But all the work of controlling the current to each coil is done by a single chip per motor. There are a few resistors and capacitors that help make the system work. If, for some reason, there is never any current being sent to Coil A (or to Coil B) then the motor will simply buzz and move in small random increments or not at all. So you might have a broken lead on the circuit board conveying motor current from the chip to the header. Or you might have a broken lead or broken resistor - the "current sensing" resistors would be the first place I'd look. You can find the schematic of the board at reprap.org/wiki/printrboard.
Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick