When you use an extender that is made by a different manufacturer than the lens, beware the possible difficulties of incompatibility. Generic extenders have to adapt to many formats so it's not a surprise that they would have difficulty with autofocus. Also with a relatively slow zoom the light input is halved with a 1.4 X extender -- isn't yours a 1.4? Many zoom autofocus mechanisms will not work when the light input is low, as is the case with some extenders.
I have Canon extenders and extension tubes, which are very different beasts. My extension tubes do not permit autofocus, but that's not so important because I use them for macro work. My extenders have glass inside and they do permit autofocus to work normally, but I have first class glass with wide 2.8 aperture. If your lens doesn't go so wide you may have problems with autofocus -- that may be the problem in your case.
Dave
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