Not seeing the issue first hand, check these items: Good proper fitting caps, hammer fits naked nipple so the two surfaces are parallel and centered. The mainspring is not week. Too small of a cap and concussion zone will not seat on the nipple. I am repeating Jaegermeister's reply.
CCI was what I was using, also tried another brand with same results. The 2nd nipple was new (6-1 mm thread).
Another thing to check is the hammer cup; sometimes you will get a spent cap stuck inside of the hammer cup and it will act as a cushion on following shots. Oh, and look twice; the last time this happened to me at the range I checked the hammer cup and thought it was ok, continued to have the problem so I packed up and went home thinking I needed a new nipple. During my cleaning I found a spent cap wedged neat as could be inside of the hammer cup.
The new nipple was square at the top as was the original nipple. After a few shots the problem began, so it’s not the caps. Inspect both nipples to see if the tip is still square and flat. The hammer cup if not hitting flat will make the end out of square and a burr will form that holds the cap off the tip of the nipple. The first hit smashes the cap to conform to the nipple then the second hit can set it off. Not an uncommon thing and you can see this once you take a careful look. If burred a couple of light passes with a fine file to just the burr on the nipple will allow a cap to seat and fire on the first hit. Then decide what to do long term.
