When I had a Witness, I found that the magazines caused a lot of failures, in my case, it was an improperly designed follower. Before I did anything, I'd order at least one Mec-gar mag for the pistol and see if that helps. You can find these at several online retailers or from the company's website...
http://www.mec-gar.com/Magazines/Witness
http://www.cdnninvestments.com/eaawi4510anm.html