Seat Not Found in Performance

Hi

I'm testing out a new facility (something I haven't done since we went live over four years ago) - the set up was all fine but now when I try and sell a ticket it's saying "Seat not found in performance" and then the seat number and performance number.

I can't figure out what I've missed, the Validate Completeness report is blank and everything was where it should be for the performance set up.

Can anyone shed any light?

Thanks

Siobhan