I washed a bi-monthly account this weekend that has about 30 gravel trucks. They travel mostly down dirt roads so have a lot of dust build up on them. I two-stepped them the same way I've been doing and it just wasn't getting all the dirt film off. It didn't do as good on the tires as it normally does or on the frames. They weren't really any dirtier than they ever are. Any ideas on what might have gone wrong?