I had a major problem with misfires in my Ultra Max a week ago.. In my opinion I cleaned it and polished it carefully EVERYWHERE but I didn't help. After a suggestion from N8T i dropped the interface, switch and pellet in some strong vinegar for a good 5 minutes and rinsed it in watter. After that it worked flawlessly.
The beveled end looks like there is a film on the edge.
I don't want to use any chemicals or cape cos above the interface where the
juice are.
Good luck