What's your mashing and sparging temp?
If you want be super careful about grains in your BK you can pick up a filter to see if it makes a difference:
filter bag | biodiesel supplies
I keep a couple handy just in case. Once my hop spider broke and so i used a bag in its place, another time i forgot to put in the false bottom on the MLT so i strained out all the grains as they came shooting out. That was a big Doh! day.