Check all the pulleys for the y-axis - especially the two for the short belts. They are probably slipping. You can use a permanent marker to make a mark on the pulley and the shaft together, and then watch as you print to see if the marks move relative to one another. Tighten the little set screws in the pulleys.
It could also be due to the motors missing steps, as you say. Does it make a strange noise when the print 'stops'? Turn the printer off, and then try moving the print head by hand, using fingertip pressure on both sliding blocks for first the y-axis, then the x-axis. Both should move quite easily, and smoothly, across the whole of the bed. If one is tighter than the other, or has tight parts, then check that the belts are straight, and not twisted, or catching on the sides of the pulley, or the frame of the printer; apply light machine oil (sewing machine oil) to the acis and cross rods; and check that the wooden end caps aren't pressing too tightly on the ends of the axis rods, and stopping them turning freely.
This is *very* common. You are slipping or losing steps in X (but not Y).
There are several causes. It's probably slipping on a pulley but lets assume losing steps for a minute. Check to make sure if you push the head around in X and Y that it feels about the same. Consider putting one drop of sewing machine oil on each of the rods. Or use a thin oil.
More likely a pulley is slipping. Tighten all the set screws. Consider marking each shaft and each pulley to see which one is the slipping one. Most likely it is the short belt which is a bit of a pain as you have to remove the X motor to get to that one. There are 6 pulleys for the X axis alone. SIX! Not 4. Make sure you tighten the set screws on all 6. Your UM probably came with a bag of extra set screws (the silver colored ones). The black ones that come with the pulleys aren't as good (although I'm still using the black ones).
Also while it is printing look at the short belt to the X stepper. If it is twisting back and forth in different directions then that means it is touching the wood in the back. This is very easy to fix if you are using the plastic spacers - just add a washer under each of the spacers to move the motor slightly away from the wood.
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