Does the LPG stove flame intensity reduces during winter?
If the cylinder is outside, it could well be too cold for the liquidified gas to evaporate fast enough. Propane is OK but butane is useless this cold. A mix of the two is often sold as propane is more expensive. A mixture can become richer in butane when used for prolonged cold periods, reducing its performance.

If this is the case, lifting the cylinder indoors for a few hours before cooking should help - take it out just before use and it should stay warm enough to get things going a bit better. Don't forget that the evaporation itself causes cooling, so it can get cooler than ambient.

This is a common problem in camping, searching for camping stove performance in the cold will give you more information.