Cold Start Problems with a 98 Bi Fuel

[confession]

I have a 98 contour. It is a 2.0L I4 CNG DOHC 16V FI Engine. It has developed a problem where it won't start on natural gas when the temp is below 40. It's acting like it's not giving it any gas regardless of if I pump the accelerator or not, no firing, and no natural gas smell coming from the exaust. As soon as I flip the switch to unleaded it will fire right up. Once I warm the car up I can stop it and then start it on CNG and it runs fine. If I start it on unleaded and right away try to start it on CNG it won't start. Is the CNG system designed not to run when it's below a certain temp? I always thought CNG ran better when it was cold because the gas is already in vapor form and thus would run better than with unleaded. The CNG system is the factory installed GFI control systems. Anyone else heard of this problem?

Any help much appreciated.

Thanks