Dogs can first come into season anywhere between 6 months and a year. Some dogs wait until their first spring before doing so.
It may just not yet be time for her to be in heat.
If she goes past a year without a heat season (and if they are light seasons you might not notice it) then you might want to check with a vet about it.
If you got her from a shelter she may have been spayed before you got her which would also keep her from coming in season.
Hope this helps you!
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