Hmm, I've not heard of something like this. (How did everything go fine if the passports weren't stamped, by the way?) Under law, at an interview the must either approve of deny your petition. You can be denied under section 212(g) (is it? I'm getting rusty on this), basically saying that there is insufficient documentary evidence to back up your claim for permanent residency. This doesn't mean that you don't get the greencard, just that you reappear (not reapply) with the needed documents. They cannot just deny you for any reason.
If what they need is a contract, then I'd suggest producing it and going from there.
Again, I have no direct experience of Schedule A, and don't know if any regulations would be different.
Best wishes.