Older Weber grill with plenty of propane won't get hot?

maybe the elements that the gas runs through are plugged up with gunk. take off the grates. take off the screen or "flavor screen" if you have one. you will see a tube or tubes that the gas runs through. It has little holes all in it for the flame to escape. Take a skewer or toothpick and push it into all of the holes. Maybe that will help. Do all that with the grill off of course.