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Do you know were the feed is coming from to the range?
I recommend when using soapy water use blue dish soap...and use alot of soap to water and don't even spray the water just let it run down onto the pipe. (pour a stream of the soapy water onto it) I have check joints by spraying and they will not show at times untill you pour the soapy water onto it.
you didn't streatch the flex line out for it to reach did you?
If the problem persists you can call your natural gas supplier. They usually do a free in home inspection for leaks. The fitting you showed me might just be your problem for it was installed incorrectly. Fittings should never be tightened down that far...the 90 itself may have a crack in it due to how far in the pipe is into the fitting.