Lately our girls are eating a lot of the grit that is put out for them. I just put a bowl of grit out this morning after filling it yesterday and it is 3/4 gone in15 minutes. I use both sizes in the mix, chick and adult grit. We have 6 chickens and they have been going through a couple of hand fulls a day recently. I am concerned this is not a good thing, and wonder what am I doing wrong?