
Well some bitches don't have a season until over 18 months of age, though more usually a first season is anything from 6 months in small breeds to about a year.
You had her at 15 months so she may well have had a season before you had her and isn't due one yet. The average inter season length is about 7 months though 5 to 10 month intervals are the norm for some breeds and bitches.
All you can do is wait, though I would try and find out from her previous owner if she has had a season ans what the pattern of inter season duration is for her Mothers lines.