What gas are you using propane or butane? As I recently discovered after refilling our Gasit system in Tarifa that the mix of gas was higher in butane and we had problems with boiler until we had used it up.
That might be what you think, but where are you getting it from?
On the continent, they use a mix of Propane & Butane. When it's really cold, the Butane does not evaporate and, bit by bit, you can end up with a cylinder full of Butane.