Resists, well if you are too low level vs the mob, then it will have a greater capacity to resist. Also some mobs have certain resists and vulnerabilities, eg, using COLD on shelled mobs and ACID on hairy mobs also use ROT on plants, would be general rules of thumb.
Some spawns always seem to have one member that resists most every spell that you throw at them, while you have been killing the other members of the spawn with relative impunity....... well I just put that down to karma
).