
I have just had a visiting bitch mated on day 16, and in fact 3 of my own litters were born to matings after day 16.
I would still keep trying until you are sure she has gone over, as some bitches are ready late, and sometimes just on a particular season. Unless premate tests were doen and show that she ovulated more than 4 days ago then she may still not be ready.
My Champion bitch on the other hand was one that didn't fancy the idea of the dog mounting and mating ehr (happy to do it to him). For ehr first litter the dog got lucky just as she was going over (day 16) 146 months later she was tried again and flatly refused. On ehr next season in May I had her with the dog from the start of the seson and she was much better disposed to him and the whole idea, though I still needed to assist as she was rather confused about who did what, but definately happy for someth8ing to happen.