This problem you post about is very common and usually easy to fix. There are many causes. After seeing your picture it is almost certainly one of two problems: A) setscrews or B) rubbing belt.
You already addressed B but did you make sure the belt isn't twisting each time the X motor changes direction? It's the motor behind the robot in this picture (back right corner). Not only might you need to add the washers (which you did) but you might also have to slide the pulley closer to the motor. It should be as close as possible without touching the motor - mine are about .5mm from the motor.
But more often the problem is that the set screws aren't tight enough (as illuminarti says). You should do at least 2 of the following things:
1) tighten the hell out of all 6 screws (six! not 4, not 5, but six). Tighten it until you are almost to the point where you are afraid you will break something. The most common problem screws are the two pulleys on the short belt because the other 4 pulleys have half as much load (but they can slip also so tighten all 6).
2) Mark the pulleys: Use a sharpie (aka permanent marker). Put a tiny mark on each pulley and on each rod. Perhaps even photograph them or make a note of how perfect your marks line up. After printing another part that slides, recheck the markings.
3) Replace the existing setscrews with the extra ones. The extra ones are supposedly better (pointier, sharper) but I haven't used mine yet as the ones that came inside the pulleys are working fine for me.
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illuminarti 18
If you've already checked that the belts aren't rubbing, then the most likely cause is that one of the X-Pulleys is loose.
Check that they are all properly tight, including the one on the motor shaft itself. I gather that UM now includes replacement grub screw for the pulleys, in case the ones included in the pulleys by default don't fix tightly enough.
I would try making a mark on each pulley and the shaft it is attached to with a sharpie, so you can see if there is any movement during the print. And I'd definitely avoid glue. Not sure how well that's going to attach anyway, but I think it's going ot cause more problems than it solves.
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