Another problem can be wet powder.
I sized some brass once (oiled the cases) and loaded up a 50 count. The first twenty were great at the range, but when I fired number twenty one, the gun just fizzled.
Sure enough on teardown, it was a squib. Some powder was not only unburned, but also black all over the bore and the barrel was caked with an oily soot.
Gotta make sure those cases are clean before loading up.