After three stuns within a certain period of time the mob will become immune to them for a short period of time. Some mobs are simply immune to stun. (most end bosses in dungeons fall under this category)

Ifrit is a bit of a different case as interrupting his Eruptions are the only sure fire way to get your LB maxed out.

Also, one thing you can keep in mind is you can mess up a casting mob's spells by running through them so that they're not facing you. Those Thunder III's and Fire III's that take a while to cast are a prime example. Mobs suffer from the same line of sight issues that players do, much to their detriment.