I've shared this with a few friends that had similar problems with Weber grills. States the correct procedure for operating a Weber grill:
From Weber:
"Thank you for your email.
I'm sorry to hear your grill is giving you problems. There could be several issues that could cause this sort of problem. Let's try to fix the problem with the follow suggestions. Do you preheat the grill with the lid up or down? If you grill with the lid down there is a possibility that you have inadvertently triggered the safety device in the adapter hose. This safety device is designed to detect leaks. If the grill is leaking, it will restrict gas flow to prevent a fire. When the gas is restricted, the grill will only reach about 250 degrees. This safety device can be inadvertently triggered if the grill is turned on too quickly.
To reset the gas safety device:
Open the barbecue lid.
1. Close the LP tank valve.
2. Turn your burner control knob to the OFF position.
3. Wait at least 5 minutes if the barbecue had been lit, then
4. Disconnect the gas source
5. Turn your burner control knob to the HIGH position.
6. Wait at least 2 minutes, and then turn the burner control knob to the OFF position.
7. Re-connect the gas source
The grill was designed for the small, disposable tanks. If you choose to use an adapter hose for the larger propane tanks, there are some simple rules to follow to ensure that the grill works properly. Please follow this procedure every time you turn the grill on and off.
Re-lighting the barbecue:
1. Open the barbecue lid.
2. Open the LP tank valve all the way and wait 1 minute.
3. With the lid open, turn the burner control knob to the HIGH position.
4. Ignite the grill by pushing the crossover ignition button.
5. Close the barbecue lid.
6. The barbecue should reach 500 to 550 degrees in 10-15 minutes.
When you are done grilling:
1. Turn all burner control knobs to the OFF position
2. Turn off the LP tank valve last. The tank should be turned off after EVERY grill use.