I had a similar problem installing a DC rubber seal/gasket in a '35-'36 windshield frame, but thought it was partially due to not installing it until several years after it was purchased. I tried every trick in the book to install it correctly, but finally got so frustrated, I ripped it out of the frame and trashed it. The next one I tried was Bob Drakes formed corner rubber windshield gasket. It was about twice the cost of his standard, non formed corner version, but it installed very easily, and I thought is was well worth the additional cost.
One thing I did find, after failing with the DC gasket, was the gasket locating channel in the frame still had remnants of the old original gasket that needed to be thoroughly cleaned out. I made a special cutting/scraping tool to accomplish that.