Did you use any lube after you cleaned the drive with WD40? Or did you expect WD40 to be the lubricant? To rely solely on WD40 for long-term lubrication is to expect too much. It's fine for cleaning off the old gunk, but then you have to re-lube the drive with fresh grease (and preferably after cleaning off as much of the WD40 as you can get to).