"Special? I don't know, just the rounds they came with when I bought it. Tell ya the truth there, hon, they all look kinda short to me."
"Hmm? Oh, yes. Aha." Oskar absentmindedly responded, uncertain if there was a rude joke being made or not. "Well, if it wasn't the ammunition, I'm afraid I'll have to open this up..."
He set the turnscrew upon the frame of the derringer, and carefully set about removing the grip, then separating the body. The screws in question were tiny little things, matching the tiny little gun in which they sat, and so it was no small effort to set them all in a little pile to the side of the counter. Finally, Oskar removed the top half of the frame, revealing the guts of weapon - it's trigger, lug, camwheel and hammer.
"Aha!" he said, as prospector might upon finding a thick vein. With steady fingers he reached down to pluck out the camwheel, and held it up to the afternoon light. "Here is your culprit, Ms. Mundee. The wheel's notches have worn quite severely. Likely the hammer was not falling with sufficient energy to strike the primer."
(OOC: piece number 5 is causing the trouble!)