We have had two seasons in the pack, very often. This has given us more than a fair share of summer litters. Basically, when one girl comes in, they all do. Sometimes its only a token dribble in sympathy from a bitch whose 'normal' ovulation is counter to that of the other ladies. One bitch, born in September, only ever actually ovulated and could be bred in the Spring although she apparently had a sympathy season with the others in Autumn.
They kept her company in Spring but matings were not successful for them at 'her' time.
I wouldn't worry but it never hurts to call on the vet if you are ever worried about anything.