Usually if you fire a cap but it fails to ignite the main powder charge;

a blocked fire channel. That is why they recommend you pick the nipple between shots.

a poor fitting cap or worn nipple which is not allowing the cap to seat properly. So the first strike sets the cap and the second one fires it.

It could be because you did not clean the breech of all lubes and oils prior to loading. This can sometimes begin to kill the powder charge.

Another cause is when you swab, you're using too much moisture on the patch and getting the fowling wet, pushing it down further into the breech blocking the fire channel. You need to run dry patches after a wet patch to make sure you are drying the barrel and breech as much as possible.

In my traditional rifles I often times will swab with pure alcohol. I find this has caused a decrease in hangfires and misfires for me. I think this is because the alcohol almost drys itself in the barrel. Also wipe the fowling off the nipple from time to time.

Another reason is bad caps. I had a terrible time with Remington #11 caps. I had 10% misfires in one tin alone. My favorite caps are the Dynamite Noble 1075 or the CCI #11 Magnum.