My new Sigma 50-150 OS wasn't focusing well on either my D7000 or the D7100. Since I'm now using the D7100, they uploaded the D7000 firmware into the lens (saying they're very similar cameras). It worked like a charm.
Your issue may be different, but the attempted solution can't hurt. If they KNOW it won't help, the obviously go with their advise.
Sigma firmware updates are the norm, as neither Nikon or Canon gives out their full system info, and Sigma needs to reverse engineer to get things totally in sync.
The firmware update works for some, but not all, Sigma lenses...
I had the older (HSM, but non-OS) 150mm f/2.8 macro that would not autofocus in live view (a detriment for macro shooting!) on either a D90 or D7100. I sent it to Sigma and they just sent it back saying that there was no fix available. (I have since sold the lens, but not on Nikonians.)
Ironically, I also have an even older Sigma 105mm f/2.8 macro without the built-in autofocus motor. Because the camera body handles the autofocus for this lens it works just fine in live view on both the D90 and D7100.
>Ironically, I also have an even older Sigma 105mm f/2.8 macro >without the built-in autofocus motor. Because the camera body >handles the autofocus for this lens it works just fine in live >view on both the D90 and D7100.
That's why, while I love my Sigmas, I never worry about my original Tamron 17-50/2.8, without the built-in motor. "Let your camera do the walking!"