I agree that if late ads are a problem, those that are late may be more prompt next time if their ad doesn't get put into the issue when it arrives late. I once tried to put together a provincial directory for another breed. I set the deadline for Jan. 25th, and the book went to the printer just days after that. Two weeks later a breeder mentioned sending an ad & when I told her she was too late, the book was at the printer, she started yelling about how "it can't possibly be at the printer, a deadline of Jan. 25th really means February 25th, everyone knows that".
Nope, sorry, a deadline is a deadline. Missing a deadline and being left out really does help to get people to pay attention and be on time the next time around.