I have not checked the power to the motors yet though.
Yup, do that. This is going to be either missed steps due to low current to the motors, or the pulleys are slipping on the stepper motor shafts.
By the way, even if the pulley set screws are tight, they can still slip if there is no flat spot on the motor shaft for them to grip. If your motors don't have a flat edge on the shafts, then I highly recommend taking the motors off and using a flat file to grind a flat edge on each shaft for one set screw to bite onto. That'll effectively eliminate the possibility of them slipping at all.