^^ Well, that scenario worked for me without issues with my 7D. Like I said I found the AF to be slow on this lens but other than that, I don't think I've even the faintest idea what might be going on in your case. Hope someone else chimes in.
i figures it out.... I had my camera set to C. FnIII "AF search off" when focus is impossible its works now like a charm.... Im really impressed with this lens the AF is super fast even at 1:1 distances!