The karcher attachments dont work as good as the HD foam lances mind, nowhere near.
I think the trouble is, people expect the car to be gleaming clean after theyve used the snow foam, I've never had this happen. Its only meant to be a pre-wash to get the thick of the grime off the car, at which it does its job.
Try different times to leave the foam on the car, 10, 20 or 30 mins, see what gives best results for you?