If you are certain your male is fertile then clearly the female is the guilty party !
If she is cycling accurately every 3 weeks/or was because most goats have stopped now, and still not getting in kid, then there is the possibility that she is infertile.
IF her seasons were eratic, then there is the possibiliuty that she has cystic ovaries, which are treatable quite easily. Your Vet will inject hormones.......usually only needed once but can sometimes be needed again. The Vet will probably recommend leaving it now until the Autumn.