That's too bad - it sort of ruins a nice day at the range. I never had problems with my Vaquero, but I did suffer the same problem with a SBH about a year ago. I blamed it on myself, since I had removed the parts to take some pictures for someone needing info, and I probably didn't retighten them enough. I purchased replacement parts from a vendor, and they also had inserts....
I'm sure that Ruger will hold up well for you - but it makes me wonder how many guns left the factory with faulty lock-nuts.