Cars Q&A

Hi All, I have Corolla 2006 model Petrol or CNG. It runs pretty good. Good Average and preform is also good but I am facing problem that is not understandable. When I start it in morning first time on petrol it starts great. but if I drive for like 5 to 10 kms on petrol and then turn off the engine and try to start. the car doesnot get started easily after 10 to 15 times I try again it start. or If its running continuously approx 50 to 100 km on petrol its working fine. But when i do same on CNG it start fine every times. I seems something wrong when its on petrol I doesnot understandable whats the problem actually.