just a shot in the dark but I have had that happen when there is dirt on either the ways of the affected axis or dirt on the scales of the affected axis. I turn off the servos and do a thorough wipe down of the ways with DENATURED alcohol - not isopropyl!!!!. The carefully and gently clean the scales and try again.
Before you power the machine down place the axes in the farthest corner from the ZZ position. Then power everything down and power it back up.