Besides the rust flakes that fall down on the slots of the burner tube which you blew off there may be a build up of fine rust particles that fall into the slots and build up over time. That tube is 15 years old and if you have never cleaned it before it may be full of fine rust particles that block the flow of the propane. Try tapping on the flue tube and see what kind of rust falls down on the burner. It is easy to remove and clean/check. You can also look down the flue to see if there is any obstructions like a insect nests. Did you blow compressed air up the flue when you blew off the burner tube?
