As you lose 1 or even 2 stops of light when using the Tamron 1.4x and 2.0x teleconverters, you camera might not be able any more to focus automatically. Most camera's need a certain minimum amount of light and contrast to get a good AF reading.

Tamron AF compatibility overview for camera' and teleconverters

As there are so many different types of camera's, Tamron has created a large list which shows you which camera's will work with AF and which camera don't.

You can download the pdf here.