Mine did this earlier in the year.
Found that one of the pipes from the LPG injector had come loose at the manifold end and one of the LPG injectors wasn't firing.
The loose pipe was found by disconnecting it from the injector end and blowing down it. If it's loose you'll be able to blow through it easily. Had to take the intake manifold off to reconnect it.
The injector not firing was going by disconnecting the pipes from the injectors one by one with the engine running on LPG. If the injector is firing you'll hearing kind of clicking rapidly. Of there's no noise it's not firing.
I'm not sure why my injector wasn't firing. I spent a good amount of time of the phone with Simon (he did my install too) and I swapped the electrical connectors around but the fault stayed in the same injector. I then took the injector block apart and moved various components to different locations but the fault still stayed on the same one. The stripped the injector block again and checked for blockages but there wasn't one.
Left it for a while and ran it on petrol and when I had time to look at it again if found it was suddenly working. No idea why.
I might have another chance to find the issue as my slight misfire has come back very recently!