as you are saying while switching back to cng it was missing... once my khyber got same problem, when i was switching back to cng petrol was not ending from carburetor, so i suggest there is VALVE of mixture under air filter near carburetor, that controls the flow of petrol, when it stops working, it need to be set at off position, so petrol should not come to carb..
you might be thinking how can i use on petrol but while switching back to petrol it will work fine...
see second pic below, make its position to exactly 90 degree i.e Vertical, and then try