Some bitches just mature a little later, I wouldn't worry about it. Physical condition does seem to account for a bit but as someone said are you sure she hasn't had a season? I have a bitch with a slightly inverted vulva, not a problem but if I didn't 'tissue test' her when she was due to come in season I may well miss it as she doesn't bleed heavily and is very good with cleaning herself when in season. Of course our other bitches are usually a bigger tell train as they do a 'train' around the house when one is in season!