John et all, the wiper arms are made to slip in case they become stuck from ice, snow or other blockage.  If tightening the nut does not secure them the wiper arm shaft hole is worn out and needs to be replaced.  There is a lock washer behind the nut, it needs to be there as it is a very special lock washer, it’s outside diameter and inside diameter are very special sizes so that it fits over the first part of the splined shaft and the outside diameter allow the cap to be fitted.  I have these special washers in a larger thickness and some times that will fix the problem temporarily.  You can order them here: 

https://www.frankcondelli.com/staging1/store/index.php?route=product/product&path=135&product_id=1326 <https://www.frankcondelli.com/staging1/store/index.php?route=product/product&path=135&product_id=1326>

Cheers,    

Frank Condelli
Almonte, Ontario, Canada
'87 VW Westy, '00 Kawasaki 250 Sherpa, "98 Ducati 750 Monster 
Vanagon/T3/Westfalia Parts and Accessories
BusFusion <http://www.busfusion.com/> a VW Camper camping event, Almonte, ON 
Lionel Trains (Collection for sale) <http://www.frankcondelli.com/trainsal.htm>