The pilot light won't ignite on my boiler -- what I should check?
It's quite normal for the jets to slowly reduce in size as soot and tar (depending on the gas being used) build up along the walls of the jet opening.

If the jet hasn't been lit for some time the soot can "fluff up" and effectively block the hole, at least to the point where the gas flow is no longer sufficient to establish or maintain combustion. In such a case I prefer to remove the jet and clean it properly, using an appropriately sized cleaning wire and solvent. If you don't have such tools you can try a short blast of compressed air instead.