I have had 3 squibs in the approx 10,000 rounds that I have reloaded.
The first squib was a .40 S&W. The bullet was pushed far enough out of the brass to engage the rifling. Not sure what the cause was, but I believe it was under charged based on the other 2 squibs.
The second was a .38 spl. I didn't notice that it was a squib until the next round was fired, the second round impacted the first pushing the bullet to the muzzle and the second bullet stopped bridging the gap between the cylinder and the barrel. I took the gun to a gunsmith, after removing the 2 bullets from the barrel and removing the other 4 live rounds, we used a bullet puller and weighed the powder that was in each round. My charge was supposed to be 3.5 grains of AA #2 but the remaining four weighed between 2.9 and 3.2 grains. I use a Lyman #55 powder dispenser and what we determined was possibly static was causing the inconsistent charges. Since I've been storing it with used dryer sheets, I have not notice a problem.
My third was a 9mm. I knew it could've happened because I was trying to use the least amount of powder to cycle the action properly.
I use a single stage press, charge the case, then visually inspect that each case is charged. I know that all of my cases had powder in them, so the only thing that we could determine was inconsistent charges. I guess it could've been the primer, but I was fairly new to reloading for the first 2 squibs so we kind of ruled out the primer.