slow switch to LPG

I have an older single point LPG system on my LSE. Generally it works fine.
Recently it seems a little reluctant to switch to LPG from petrol.

I need the engine warm on the dash gauge and even then often the first attempt fails and when I lift off the throttle completely to swop over there is nothing when I put my foot down again. If I'm quick I can flip the dash switch back to petrol and back again to try and swop over once more. If I'm not quick enough or haven't realised I've switched it just stalls.

Once running it is fine, idles properly etc. It does seem slightly more sluggish on LPG than petrol and I don't think it did before. It's not much though.

Any thoughts?

Sounds like the lpg system could do with a service, new filters. Switch over can be tweeked if you have the software/ cable for your setup.
Could also be a problem with the hot water supply to the vapouriser. make sure your cooling system is full and that the water pipes going to the vaporiser are getting hot. normaly top = hot in. they can get airlocked. Vapouriser can freeze up if its not being heated this shows up as frost on the outside.