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Chris,
What exactly is the problem ?
Please post an image and the EXIF info (shutter speed, aperture, AF mode Exposure mode etc) otherwise its impossible to offer any reliable help.
If the problem you are having is at long focal lengths eg at 400mm then its quite possible that the problem is image blurring caused by lack of stability and too slow a shutter speed ?
The 35mm Effective Focal Length of that lens is 640mm and this requires some developed techniques to manage.
Lots of practice !
You simply cannot hand held at these focal lengths and expect to (reliably) get anything useful except at extremely high shutter speeds.
A substantial tripod is required, a minimum shutter speed of at least 1/1200-1/1500s, mirror lock up with a 5-10 second delay, a remote shutter release and no wind or straps attached to the camera - as a starting point
A regular photographic tripod will not be sufficient except in the very best of circumstances and with some luck.
You really need a very rigid tripod for good results at long focal lengths
I am not familiar with that particular lens but most (not all) Sigma lenses perform much better stopped down at least 1 but preferably 2 stops or more.
What Exposure and AutoFocus mode are you using ?
Generally you would use a single point AF mode - I normally use centre spot.
However depending on the metering mode you are using this can cause problems with exposure - eg If your subject is a bird with a black eye and you use the eye as the AF focus point, the exposure might end up over exposed, or if a white eye under exposed !
So every situation is a bit different - depends on what your problem is ?
It is quite common to hear of people experience this problem if this is their first experience with a long telephoto lens.
It has been reported that occasionally some Sigma lenses (and Canon/Nikon too) have a back or front focus problem straight from the factory - this can be verified by placing a ruler at 45° and focussing at the half way mark and then looking to see where the best focus appears in your image.
If that is the problem then they can usually fix that locally by the distributor or swap it.
The shorter focal length success you have had may well be because of the fact that wide angle lenses are much more forgiving with exposure.
They will also provide a much more saturated image compared to the reduced contrast that you will get from a telephoto.
Atmospheric haze is also another problem commonly encountered with telephotos.
More info would be more useful - hope this helps for starters.
Cheers
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