I've been told by some that I can't hand hold my 135-400, however I've never had a problem with it on nice days. The main thing is I usually used ISO400, giving me a little faster shutter speed.
I'd have to see the pic's to try figuring out the problem. With that lens you may even be just a little too close
However, if you do have a problem with it Gentec (Canada's Sigma distributor) should be able to correct it quickly. For the best results they really need the camera though.
However that might not be too bad... for you. They're located in Ontario, so there's a small chance that they're close to your house. If they are I don't imagine that they'd mind a quick phone call, and they may even say to come on by and get it fixed fast.
Here's their contact info.
I'm going out with the camera for a few hours, so please don't expect a response from me right away