If it shows a flashing green light on switching it over to gas, then the conditions for switchover haven't been met (minimum temperature of vapouriser and / or minimum RPM for switchover and / or minimum time after starting for switchover).
Obviously, with a bit of driving these should soon be met. You should get a solid green light once it's running on LPG.
If something goes wrong at that point, you'll get some warning beeps and it'll go back to a flashing green LED and switch over to petrol.
If it never switches over to a solid green light and you don't get a warning, something is preventing it from switching over in the first place (no RPM signal or insufficiently high vapouriser temperature).
It'd be well worth getting hold of the software and having a look at what's going on. You're pretty much blind without it when it comes to problem solving. The software is free and connection leads aren't that expensive.