Having a bunch of Fire weapons in Frostburn Canyon has made me notice sometimes my fire weapons (even a high powered sniper rifle) deal absolutely no damage or extremely low damage (3-4 points) on enemies that resist fire and sometimes they deal lower but still moderately okay damage. I've hit critical headshots to no damage (or moderate damage) on resisting enemies too.
Is elemental resist a % chance? Does it only effect shots that "proc" and hit with an elemental effect? Why are some of my shots getting through while others aren't?
In Borderlands 2 enemies will take significantly less damage from weapons utilizing elements they have a resistance to.
Quote from Borderlands wiki:
Elemental nerfing - Whereas in Borderlands an elemental-based enemy (e.g. a fire skag) could be killed with sheer bullet damage from a similarly based elemental weapon (e.g. a Hellfire SMG), in Borderlands 2 enemies will take significantly less damage from weapons utilizing elements they have a resistance to.
Certain enemies do have a chance to resist certain elements. For example corrosive spiderants will completely nullify corrosive damage taken occasionally and do take far less corrosive damage over all. So you'll definitely have to switch to another element to reliably fight them. I'm afraid I don't know the exact % of the time that they will nullify, you'd have to examine the game's code for that.