
My original bitch was like clockwork, 9 months to the exact day - I could mark her on the calendar and it would be right. Sometime after turning 4 she went about 13 months between seasons for no reason we can think of. We actually wanted her to come in on time hoping that it would bring our youngest into her first season a bit early, but no joy...
After that 9 months like clockwork again. No rhyme or reason.
Our latest bitch who is always on schedule (usually 2 weeks earlier than 9 months) was 5 weeks late this last season - probably because we wanted to breed her to a foreign male we planned to travel to and it suited her to try and mess up our plans ;)
I wouldn't worry too much as long as they're healthy and happy.