I assume this is in a caravan and has not been serviced recently.
One of three causes:
1) Air in the gas line. Occurs when changing cylinders or removing cylinder for winter storeage.
Remedy: Purge as much air as possible out of the gas lines via the cooker (Usually next to the fridge) and any other gas heating appliance. With some patience keep the gas knob pressed in for up to 5 minutes whilst still trying the igniter. If your lucky you will hear a 'pop' when the ignitable gas gets to the burner outlet.
2) Igniter not working: You have to remove the fridge unit to get to the igniter. Only very rarely can you access it by removal of the vent grills on the outside. Look for poor electrical connections to the igniter, disconnected or damaged feed to spark generating transformer or failed transformer. Can you hear the igniter sparking?
3) Blocked or obstructed burner tube. The lit gas has to rise up a metal tube. If the tube is blocked by rust particles or over-wintering 'creepies' etc. the gas may be deflected away from the igniter.
Removal of the fridge to get at the burner tube is the only option
NOTE:
Do not remove the fridge unless you are competent with LPG gas connections. These will have to be undone to get it out and more importantly tested for gas safety afterwards to ensure you have an undisturbed nights sleep on your next trip.
Don't forget there will also be a 12 volt and a 240 volt connection to break and re-make
Cledlyn
July 2008