Check the belts — are they in good condition w/ even tension? Measuring belt tension, squaring and calibration
Are the pulleys secure w/ both set screws in place? Checking Pulley Set Screws - Carbide 3D
Are the eccentric nuts properly adjusted to hold the V wheels w/ the correct tension? Tightening Eccentric Nuts - Carbide 3D
Is the wiring in good condition and all the connectors secure?
Power down and remove both belts (label them so that you can swap them side-for-side and end-for-end) — if nothing is obviously wrong w/ the belts, put a bit of tape on each pulley so as to see their rotation, then power up, connect to the machine and initialize — the Z-axis should home as expected, then the X- and Y-axis should begin homing, the X-axis should home as expected, while the Y-axis pulleys should turn until they time out — do they turn evenly and in synch?
Let us know what you find out at support@carbide3d.com and we will do our best to assist.