I recently had this problem, and discovered that the rods were shifting in the ends causing the belt to slacken off, a little dab of super glue fixed the problem.
Best posts made by erniehatt
Point of interest
Had a problem arise last week, one of many to be truthful.
After printing a test cube I found I needed to adjust the X and Y E steps, no problem. But when I went to print the cube again on the following morning, the Z axis started running at less than snail pace.
After swapping motor drivers and axis leads, I determined it to be either a control board or Marlin problem, most likely Marlin as I had made adjustments there.
So looking in the area I had changed, ( this I might add is after a few days of searching else where) I found the problem, a simple little typo error, a Comma instead of a full stop, 80,78,52, 4000,837.00
Instead of 78.52 I had put 78,52, it was reading the 52 as the Z axis speed instead of 4000. a simple mistake that could send anyone to the funny farm.
Well I downloaded the update, and of course I immediately tried to see if they had fixed the bluetooth problem I was stewing about, and guess what it worked a treat, thankyou Guys, well done, I like the interface as well.
Merry Christmas and best wishes for the New Year to ALL. Ernie