That really shouldn't happen. Becker's sheaths are glass-nylon filled so they have been known to cause this problem, but the Izula sheaths are just plastic-injection molded sheaths. I've had the same problem though and talked to Jeff Randall and he said there's no way the plastic used for the sheath should be hard enough to dull an edge at all, and suggested that there was probably some debris of some kind stuck in my sheath. This was easy to confirm for me because it only dulled one part of the blade--by the sounds of it, it's dulling the entirety of the blade for you? Shine a light in there and see if you can see anything visually.
I'd also suggest oxidation though, and also how much dulling are we talking about? Sometimes when I sharpen a knife up to hair popping and let it go for a while, and try to pop hairs with it again, it really doesn't. If it's not even paper slicing still that just sounds very weird and I'm thinking that the knife didn't come very sharp, but rather had a burr (a burr will seem very, very sharp) that has since worn off. The soft plastic of the sheath could have gripped and tore this burr off--though that's a pretty long shot in my opinion, I know some people use corks to debur very thin edges, but I just can't imagine plastic this soft wearing it off unless you've taken the blade out and reinserted it thousands of times.
In any case, ESEE should take care of you, but might suggest flushing the sheath out with water first. I tried that myself and it did no good, getting a new sheath fixed the problem.
Edit:
Oh, and the fit and finish on the Izulas aren't quite as nice as the rest of the line-up. The Izulas are kind of "economy line". I'd suggest an ESEE 3 to try out, most of the models are pretty "beefy" but the 3 is quite thin in the blade stock and the handle and feels pretty light for its size.