B+W Circ Pol and 20D Autofocus issues

Hi Everyone,

My 20D hunts a fair bit and is not able to autofocus lock at times when used with a B+W Kaessman MRC Circular Polarizer on 24-70 2.8 (using center focus). This is even when it is aimed at very high contrasting things deliberately to aid focus. Rotating the Circ Polarizer may sometimes result in focus lock (but not always) and may not correspond to the ideal polarizer position where the sky is 'bluer'. Is this common or unusual? There are no focus issues at all (even in the dark) without the Circ Pol.

Your comments are greatly appreciated.
Thanks.