it turns out when the assembly manual tells you to follow these blue arrows, you need to be very careful to actually do that instead of following the red arrow
if you follow the red arrow, it turns out that what can happen is that the print head will move normally and the printer works fine for like three or four days -- until the surface texture of either the belt or the little plastic nub you routed it around changes enough to seize the machine up and make it skip steps
either way, after rerunning the belts (again) it seems to work normally, so that's a relief
