Hi...I am having the opposite problem. I have a duel fuel Ford Sedan that worked fine, the one day it started fine on Petrol, but wouldn't switch over(heard no click) to LPG. I discovered the LPG fuse in the instrument panel was blown, so replaced it, by disconnecting the battery.
I started it on petrol, then it clicked over to LPG, so thought it was fine and went inside. The next day, I tried to start it again, but it wouldn't switch over again and found the fuse had blown again.
The LPG fuse is a 7.5, so I tried stronger fuses - the 10 and 15, but they blew too. I asked the guy at the ford spare parts and he suggested it might be something electrical in the LPG circuit. A loose wire, maybe? Next, I am going to take a look at the switch and see if it is the switch - I hope it is not one of the Selanoids, as they are more expensive to fix.
I have the Ford manual for my model, but it doesn't have a great deal about Duel Fuel system in it. Process of elimination I guess, starting from the least expensive and hoping for the best - might have to take it to my mechanic if it starts looking complicated - electrical stuff isn't my forte (little ford joke there).
How did you go with your problem, did you work out what it was? How to fix it?