Mine used to do that when I used a hammer extension. T/C rebuilt it 2 times and it still did it. The 3rd time they rebuilt it they tolded me not to put the hammer extension back on it because the spring was marginal with some types of primers and the spring became weakened with the extra weight. Eight years later there is still not a hammer extension on either frame and I have never had another misfire.
Some will tell you this is hogwash because they have used them for years, but I believe what I experienced.
