100:1 Geared nema 17 wont turn

I'm designing an extruder with a 100:1 NEMA 17 motor. When I went to test the motor yesterday it it wouldn't turn

I took the gear box off and the motor itself would turn, but the gearbox wouldnt. When I dismantled the gearbox, I didn't find anything unusual, so I'm kinda stumped why it won't turn.

I'm using a RAMPS board with DRV8825 drivers. Here's the motor I'm using:

[www.omc-stepperonline.com]

Any ideas?