why are my burners not lighting up properly?

[mosaica]

I've got a gas hob. Two of the burners do light up but the flame is very weak, it's pretty useless for anything other than simmering food. This started a few weeks ago and i thought maybe some water had got into them, but they are dry as far as i can see. What do you think it is?

probably dirty. Boiled over pans, or fat splash from frying, will do it.

If you have a dishwasher you can put the tops in there, if not, scrub with detergent and rinse well.

Thanks, will try that. They don't look too dirty but they probably are!