Extruder motor just "rocking" back and forth a little

I'm in the configuration stages of a Folgertech Kossel 2020 using a RAMPS board (1.3 or 1.4, I'm not sure). I've got the X, Y, and Z extruders working fine, and I can load a gcode file and the extruder seems to make all of the right movements except the extruder doesn't advance the filament. Whether I try a normal print job, or manually send a "G1 E15", or use Printrun/Pronterface' "Extrude" button, the serrated extruder wheel just kinda "rocks" back and forth ever so slightly. I can manually push the filament through, so I know that the extruder is hot enough.

The extruder stepper is plugged into the RAMPS board correctly (and oriented the same way as the X, Y, and Z connectors). I've got the driver board pot set to about 0.55V.

Anybody know where I should be looking to find the problem? Is this likely a driver pot setting? Is the driver board probably bad? Something else?

If the extruder moves correctly without filament, but doesn't move with filament, it isn't strong enough.
There can be sevaral reasons for that, like the idler wheel is too tight or the filament path isn't straight.
Did you check the grub screw of the drive gear on the extruder motor?
Depending on the max. current the extruder motor can handle, you can also rise the Vref of the driver.

Another thing is the firmware: Did you set more than one extruder in configuration.h?
Or messed up the optional extruder mixing stuff or the like?

Found the problem. One of the wires to the extruder stepper motor got pinched enough to sever all of the strands. Soldered it, and it works fine, now.

In case someone else comes across this thread in their troubleshooting search, another thing I tried was using "M203 E" to lower the feed rate just in case I was trying to drive the motor too fast.