I had a problem with my pressure canner this year not sealing. I tried everything I knew, rubbing a little vegetable oil on the seal, dampening, even turning off the canner letting it cool completely then starting the sealing process all over. Still, to no avail. Even a new seal didn’t do the trick.
I then bought a new pressure canner. The next morning when I got ready to reprocess the entire batch they were all sealed. Yes, sealed from heating the canner twice until the water boiled and steam escaped (even though the canner did not form a seal).