It might well be a jealousy issue , or your Tortie deciding that she is the Boss Cat , and keeping your Boycat in his place.
Pouncer , my first Cat was like that with Mr Scrappy at first , but eventually they were my Damon and Phythias , and Dear Companions, as they are now at the Rainbow Bridge.
So I wouldnt worry about this, they have the way of working themselves out