How to light a Thetford Gas Hob

On our Bailey caravan, press and hold the control knob and ignite the gas (fine) - hold for about 15 seconds then release the knob.  When gas hob fails to stay alight - try again, and again and again. My wife asks if I can hold the knob in whilst the potatoes boil. Has anybody else had this problem?  My dealer says its fine.  Note: all windows, vents and doors closed.