One of my horses keeps pulling on her bit, how can i get her to stop?

When one of my horses canters she pulls on her bit, and puts her head really low. Its hard to make her keep her head up, she doesnt buck, but it just gets annoying and shes trained for gaming. How can i get her to stop pulling on it? Oh and P.S my reins are the same length they have always been and if i loosen up she goes faster than i ask of her.