1.Start from checking an electrical connection on valve between vaporiser/reducer (One which lets liquid LPG run into reducer). If both connections (+12V and ground) are connected,
2. Go to back of car, to LPG tank, where another valve should exist. It is built into multivalve located in LPG tank (most often in luggage area or in spare wheel gap). That should be similar valve, running on the same circuit, as that one near (or in) reducer. They should than work at the same time.
If they are both connected, and not clicking when system turns into LPG,
3. check electrical circuit between ground connector in reducer valve and engine's body,
4. next chek +12V circuit between reducer connector and LPG controller plug (don't forget about pulling of ground cable from battery!)
If both valves are properly connected, electrical circuits between valve(s) and ground and valve(s) and LPG controller are ok, but they're still not working , Your LPG controller is fried unfortunately.
If both valves are connected, both are clicking, and it still doesn't take effect,
5. Find LPG pressure sensor and check it is connected properly. If it is, next
6. Check for LPG pressure in system, between reducer and injectors. If it's too low, engine will not run on LPG, and will switch back to gasoline.
If LPG pressure is ok - LPG pressure sensor seems to be damaged, or LPG controller is gone mud
If LPG pressure is about nothing - Your multivalve in tank may be damaged or jammed, liquid gas copper connection under car may be bended or hitted with some rock, Your reducer may be damaged.
Of course I believe, Your LPG tank is not empty!
I think that are all possible reasons.
Good luck with fixing the problem!