The more I play Cassiopeia, the more I find every so often twin fang doesn't reset like it should. Im not sure exactly why it does this, but I suspect it happens whenever twin fang hits the target at the exact same instant the poison damage ticks or renews

Is it possible you are casting E too quickly after Q so your Twin Fangs hit before the poison is applied?

I've been trying Cass in the jungle... and having a bugger of a time getting the Twin Fangs cooldown reset. I'd even have Q AND W on a target, having waited a half second or even a second, pop E, and still get the 5 second cooldown.

Doesn't seem quite as much an issue in the lane, something about the jungle creeps is bugging the **** out of E.