I've been filming for around 3 years, and my fisheye has always been a little off, you get used to it. It's pretty much impossible to get a perfect circle, so most lens look like that. Depending of what fisheye you have, it will have two lens, take out the thicker lens and it should fix the unevenness. The only problem with this is that the fisheye won't have as much of an effect as before. Could you post a a video of yours so I can get a better idea of what it looks like?