could be:
dead stepper
dead stepper driver
broken stepper wire.
vref not set
to much phyiscal resistance in the axis
presuming you have a board that uses pololu modules....
While everything is off swap X and Y motors, does the issue move or stay?
While everything is off swap X and Y pololu driver, does the issue move or stay?
Have you set the vref pots?
With the belt removed does the X axis move freely?
