Hello, newbie here.

Iv'e had my Contour since February and it's been working fine on both gas and CNG until June. After a fill up I got on the highway and after about 10 miles it switched back to gas like it usually does when the tank gets empty. I stopped the car and tried to put it back on cng a few times and I got at most 4 miles. Now the strange part, ive realized that i'm only able to start and run it on cng (for 10 miles max) when the weather is relatively cool (below 80 degrees) I can travel the furthest in the mornings . I often travel to San Francisco and there I can get it to run a bit on cng before it switches back to gas but down here in L.A I can't even start it on cng. Could elevation have something to do with it? What could be triggering the system to keep switching to gas?
Car has 85k.

Add info: Before it stopped working the tank sat empty for a month or two since i was in a location without Cng stations.

Thanks and forgive me if this has been already discussed (I searched the forum and didnt find anything similar)