LPG cooker oven wont stop ignition?

Hi there,
I repalce teh rangemaster which is currently not happy with working, i have got a LPG cooker, but it also has a problem.
When i start the oven up, the ignition keeps going even though the flame is up and running.
I have had to only use the 4 hob's and disconnect the electric and use a lighter on the hobs, i cannot use the ovens though and the ignition always going could cause an incident.
Any ideas why the ignition doesn't stop?
many thanks