Sometimes cobwebs get in the burner tube.
- Remove the black tube from the metal burner tube, there is a funny U shaped clip...don't lose it!
- Clean it out with a bottle brush, then blow it all out with air
- Reattach black tube to burner with the clip
That should do it. If you keep having issues, you could try hooking up a new tank of gas, maybe the old tank has a lot of oxygen in the bottle.
If you still have problems, then you might want to give Weber customer service a call since something like that should still be covered under warranty with it being only a year old.