How can i get oven working properly?

Have fitted 2 new flame safety devices in both ovens of my Leisure rangemaster 1100 cooker. One is working properly but the other oven hardly ever works. The oven lights but doesnt get much higher than the pilot light. Both FSDs were from same firm. This is the second FSD have put in that one oven. Can anyone please offer any advice. So expensive to keep fitting them in.