Squibs are usually caused by a malfunction in the loading process where no powder, or a very light powder charge is dumped into the shell. The problem with Squibs is that they have just enough punch (sometimes) from the primer to lodge the bullet into the barrel, still permitting another round to be chambered with a severe barrel obstruction. We all know what happens next.
The problem the OP describes is really more of a hang-fire, a delayed ignition of the powder by the primer usually because the powder is damp or there is some problem with it. Most factory ammo already have sealed primers, some have some kind of sealant where the bullet is seated. In that case, further sealing the cartridge with nail polish probably won't do much. But, if I were going to be carrying a gun in a very wet, jungle environment, I might take some extra precaution and put an extra seal on the primer and around the bullet. Just don't over do it, and there shouldn't be any problem.
