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The filter in a gaseous fuel system has become restricted.

This is the most under-serviced part on a converted vehicle, and the reason is administrative rather than technical. A gas conversion is usually aftermarket, so it does not appear in the vehicle's service schedule; the car changes hands, and nobody who owns it afterwards knows the filter exists. A conversion with several previous owners has very often never had one changed.

LPG carries oily residue and heavy ends that the filter is there to catch, so restriction is a matter of time rather than of anything going wrong. It builds gradually, which is why the symptoms arrive as a slow decline nobody can date: worse economy, hesitation under load, longer changeover times, and eventually the system giving up and staying on petrol.

The diagnostic signature is a pressure difference. Healthy tank pressure with low pressure at the regulator inlet means a restriction between them, and the filter is the first thing in that path worth suspecting.

It is cheap, and it is worth asking about before anything expensive is quoted. A vaporizer, a regulator or a pressure sensor can all be blamed for what a filter is doing.