Ok so I wanted to do a little bird type photography, the options for me were a slow 200- 400mm lens or a quick 70-200mm lens with a teleconverter, I go with the option of the 70-200mm Sigma f2.8 lens as I figure it will be more useful as an all round lens, the lens is great on its own it auto focuses on moving targets very well and takes very nice photos IMHO, I buy a kenko 2x teleconverter and now the lens won't auto focus properly it simply won't hold focus it will go into focus then drop out, it just can't hold a target.

So where have i gone wrong are 2x teleconverters just too strong even for a f2.8 lens or are kenko crap, I am using a d3000 could this be an issue as well?