Something crawled into one one of the gas feed pipes over winter and is now blocking the feed.
This happened to me last year and after taking the entire thing apart and still not being able to fix it I dug out the manual and one of the 2 or 3 things listed in common problems was exactly as I was experiencing and you are describing. There were clear instructions on how to clear the obstruction - it basically involves taking the thing apart, sticking a bicycle pump up it and blowing whatever it is that is blocking the feed.
Obviously, gas plumbing is not to be trifled with so if you do go down this route be absolutely sure you know what you are doing and be absolutely sure you put it all back together exactly right and tighten everything. This was described as user maintenance in the manual I had, so as long as you are carefull you should be OK.
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