Whats changed if anything? Was it working properly then all the sudden stopped? Any new firmware uploaded? Is it a new model your trying to print but the others work? Any setting changes?
My initial thought was if nothing has changed and the Y-belt isn't skipping; has the machine been running for hours at a time and works for a while once cooled down?
I've had motor drivers worked to the point of getting too hot and they'd skip steps - a cooling fan over the control board fixed the issue for me.
