I don't know where to start in order to solve this problem in the short term. I tried searching but did not see this scenario addressed.
I've been using my D7000 with Sigma 8-16MM lens, with attached (hotshoe) Neewer flash for about 3.5 years.
Everything was working fine, the flash was acting up a year or so ago, so Neewer sent me a replacement and I had also ordered a new one in the meantime as I needed it for work.
So now I have 2 identical Neewer flashes and everything was working fine. A few months ago, the autofocus was sporadically not working. I would have to try autofocus 2-3 times before it would focus. Other times, it worked fine and the lighting situations were not very different at all.
About a month ago, I did some experimenting to try and find the problem. The camera and lens have NO problem with autofocus in any lighting situation with the flash setting OFF. So I thought maybe it was the flash unit I was using, so I switched to the identical Neewer flash. Same thing. About half the time, autofocus will not work. I was having to rotate the camera on the tripod to get autofocus, then swivel camera back and press button all the way to get the photo.
It seems to be getting worse and I just came from a very frustrating photo shoot. If the camera will not autofocus on its own, I have to turn flash off, then no problem with autofocus, holding button halfway down and turning flash back on with other hand, THEN get the shot.
I had to turn the flash off and back on about 10 times on this shoot. I can't go on like this....
Lens and camera are set to AF and like I said, this problem started after a long period of working absolutely fine. I also looked at hotshoe and it does not appear to be dirty or damaged. My next step is trying the flash on a remote tripod. The problem is I can only try this at home and the camera doesn't always have this autofocus issue. Only when it's most inconvenient, of course!
Is it a Nikon D7000 issue?
Sigma issue?
Flash issue?
Thanks in advance.