If the bucks in your area are well educated on grunting they might relate that sound to a past experience. I would have tried the same thing. Are you grunting softly? Once the rut is in full swing, then you could get more aggressive. Are you using an adjustable grunt tube? If so you could try to imitate an immature buck because maybe there's a monster that none of the other bucks want to mess with!! Stay positive!!! after all, you are seeing shooters!! Good Luck