Tokina 11-16 2.8 AF problems...help!!!

Hello!

I bought new tokina 11-16 2.8 for my canon 40D... At first I was impressed by this lens because it is really sharp... But then I want to use it in club in low light condition with flash... When is dark AF beam stars working(red light in speedlite which helps to find focus....)...there is a problem...when AF-beam works I always gets back-focused photos...I do not know why because I newer had problems with my tamron 17-50 2.8.... Then I tried to focus with built-in flash with tokina and it works!! ... so I do not know where could be a problem so I will be grateful if anyone who owns tokina and canon body can told me what could be a problem...

So here are few samples:

At good light(sunny):

In club(low-light):

And comparison between tamron 17-50 @ 17mm f2.8 and tokina 11-16 @ 16mm f2.8

focusing with built-in flash:
Tokina:

Tamron:

And focusing with speedlite 430ex mk2 (AF-beam)
tokina:

(back focus!!)
Tamron:

So please help!!!