In reply to mh554:
It's usually the jet (the little thing that screw off at the top) which is partially blocked. Use a little pricker to clear it.
If you have none, take a fine metal brush, remove one of the wires and hold it with a pair of pliers and use it to poke the little jet.
If that doesn't help, you might need to ask the manufacture or the stove people (spiritburner.com)