Hi. Should I be worried that my bitch is still not in season at 15 months, her mother and grandmother both come into season by 12 months . She has lost all coat and I hoped that she would have been in season by now as she lost it around 13 months. I have never had a bitch not be in season by now.

Could she of had a silent season ?

Mine was similar and had few seasons in the 5 years before she was spayed. No health problems either observed physically on spay and no other  problems previously showed. 
It was just her.
I didn't intend to breed from her so just looked at as an easy life with no season s to manage.  More tricky if u intend to breed of course

I don't believe she has as I have a great stud dog and he has not showed any of his signs if a bitch is in season. I do hope to breed from her next year as she has passed all her health tests.

In which case  others may have advice for any testing you may be able to do as you intend to breed.

It's not uncommon in my breed for bitches to be 18 months or older before their first season, in fact I know 2 that first came in over 2 years old. I wouldn't worry just now.

Breed? because I was surprised when, by 11 months or so, my Whippet had shown no sign of coming into season - and I had an entire male at home too who showed no interest in her at all.   I was waiting on a first season as my then vet didn't want to spay when I first took her in to book that, at 6 months.  My entire boy wasn't well and didn't need to have an in-season bitch under the same roof, even if separated.   I talked to her breeder about this and she said her mother, and both half-sisters didn't have their first seasons until around 15 months.   It seems she was going to be the same but I'll never know as by 11 months I asked my vet to go ahead with the spay.  I had, incidentally, no intention to breed from her.

It can be up to 24 months before a bitch can come into season for the first time.

I had a GSD who had her first season at 18months. My BSD was almost three years old

Hi. She is a GSD, the only sings of her thinking about a season is loosing her coat and. I bred her mother and auntie and have had her grandmother since 8 weeks and have all been by 12 months, all the other females in the mothers side have all been by 13 month (friend bred them)

I have had two bitches (unrelated) whose first seasons were at 4 years old.  Both lived with males who never showed any interest. Both girls mated at their first season and had normal litters.  One went on to have two more litters and the other has had her first litter this year.  They were boxers.  I thought they were both barren!  Shows how wrong you can be.  Over almoat 40 years in the breed I have found that the girls usually come in between 8 - 18 months.  Mother Nature sometimes has other ideas.

My girl was 26 months with her first, she came in second time 12 months later when she had a litter, she had 8 lovely puppies.   When I asked my vet about testing before her first season he said it would be costly and suggested waiting, which we did and with that she came in season.