I had the same problem with the snow,, but my wipers hit each other and got totally mangled. After replacing the wipers I noticed the new one on the near side didnt move. I took the full arm off and noticed the stump that comes out of the bodywork which the arm bolts to all the teeth had been ground out. my only option was reposition the arm and make sure the wipers didnt touch when operating then tighten the nut on the spindle as tight as I dare. To this day I have had no more problems with them.
hope this helps
Andy, March 2010