Impatient with my Impatiens

Recently my Impatiens have gone from a full and healthy looking plant to looking very "stemmy" and stringy. Not a lot of flowers like before. I haven't changed my routine with them and I'm wondering if there's something more I can do to bring them back to looking full and healthy again. What can I do?

I remember reading a few years ago that downy mildew has become a real problem with impatiens in the U.S., destroying whole greenhouse stocks. It looks like maybe this is what got your plants?