UM2 - Extruder motor will not move

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I really hate that this is my first post, haha.

Anyway, I just finished building an Ulti 2 that's pretty DIY, although I did source and build it as closely to the official Ultimaker BOM as I could. Today I turned it on for the first time, ready to troubleshoot and calibrate, and everything is working mostly quite well.......except that the extruder motor absolutely will not be moved by the printer.

It's worth noting that the extruder motors (I have two) are actually different from the XYZ motors, with different specs, though by all accounts (I think) they should be compatible with the Ulticontroller. If I plug one of the XYZ motors into the E1 port, it's controlled just fine. Conversely, the extruder motors will not be moved by other ports. So something is clearly up with these motors, or with the software driving them.

So my question is: where do I go from here? Should I be looking into some other firmware, or should I be looking at some other motors? What exactly should I do next?

Let me know if there's particular info I could post to clarify the situation.