If there is a build up of burnt on food on the burner caps the spark ignition may not work or the gas will not be able to pass through the burner cap to ignite.Please ENSURE THE HOB IS COOL before starting any cleaning.
Remove any pan stands and simply lift off the flame spreader that looks like a cap and the burner cap. The components can be washed in hot soapy water. You can also clean them with the Hoover hob cleaning products available from the spares section of the website. Let them dry then re fit to the hob and try lighting them.