A lot of maternal line's seem to go dormant and in my opinion it come's down to sire selection by breeding to 1 or 2 that don't suit and keeping the fillies by them and breeding on to unsuitable sire's again. They seem to come back to life when the family find's a sireline that suit's it and so the merry go round begin's again until that bad choice or unlucky choice happen's and dormant again.