Is your WDH shank a solid square bar or is it shaped like an I-beam?
I recall a recent thread on here where somebody had a similar problem... The shank looked similar to this one:
![](http://www.etrailer.com/Merchant2/graphics/00000001/pics/R/P/RP54976_500.jpg)
but did not have the reinforcement where the hitch pin went through, so the drawbar was loading the center of the pin only, not placing the load at the edges of the pin. The pin is VERy strong in double-shear like a traditional shank puts it in, but weakest in bending which is what was happening to that other person.
If your shank looks similar to the one I posted, but doesn't have the casting reinforced out to the edges, that could be the problem.