Rimfire cases are drawn from a thin piece of brass and formed with a hollow rim. A priming compound is then forced into the case using centrifugal force, where it is charged with powder and a bullet is seated in the mouth of the case. The case is then crimped around the bullet to ensure sufficient push and pull when the round is fired. When the firing pin strikes the thin brass rim of the case, the hollow rim is crushed and the primer is ignited.The process is not infallible, and it is possible to get rounds where the priming material has not been properly forced into the rim. Sometimes, it just takes a second strike in the same spot or different spot to ignite the charge; but, more often than not, it is simply a dud. And rimfires are statistically much more likely to result in a dud than centerfire ammunition. This is one of the principle reasons why rimfire is considered less reliable than centerfire ammunition.
This point was driven home to me this past weekend when out shooting. We finished up a box of the Federal Auto-Match .22 LR and started into a new box when we immediately ran into problems, experiencing 3 or 4 duds in a row. I tested the rounds in different weapons to see if it was just a possible issue with light primer strikes, but, no, the rounds were definitely dud. Although I didn't keep track of exact numbers, we were getting two to three duds per 10 round magazine and sometimes more. Unfortunately, I didn't have any other boxes of .22 with me. On the bright side, my kids and I did get practice with clearing malfunctions from a few different .22 LR weapons.