Contax 645 does not advance film to frame 1

Hi all,
After searching a bit, I can't find anything related to this issue :
I am trying a new back on a contax 645 but the back is not advancing the film to frame one, the viewfinder show 00
if I change to another back wit an insert loaded, it will advance the film.

This is really annoying as the back looks like new...

Thanks,

Luc