lpg?

The LPG switch inside my car shows a red light when switched to LPG...and uses up my petrol(have ran out+had to be rescued lately!)
Any suggestions please.?..I've had to be rescued lately when I ran out of fuel....but car wouldnt start again on LPG even though I know there was plenty in Lpg tank.

I have a Vauxhall Vectra Estate X reg