One of two things:
1. slipping X or Y belt
2. Stepper driver voltage under or over
If the belts are slipping you will need to obviously tighten the belts. There should be no slack and when you poke it get about a half inch give with a little pressure.
Stepper driver voltage is a function of the adjustment POT on the stepper driver board attached to your RAMPS. I read the printer manual and it gives examples of where exactly these POTS should be positioned. Reference and make sure they match the diagrams in the printer manual.
Alternatively what I do .. be careful with this or things will fry .. is take your multimeter and touch the voltage adjustment POT with one probe and the NEGATIVE pole of the RAMPS power terminal block. A good voltage seems to be agreed upon at 0.4 volts. I adjust all my steeper drivers to this and has always worked like a champ on my past three printers.
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