Why does my gas canister cool down when I’m using my stove?

This is because the gas is evaporating inside the canister cools the remaining liquefied gas inside the canister. The lower the temperature of the fuel in the canister, the lower the gas pressure. You can lose gas pressure completely if the fuel falls below freezing (for pure butane) or below around minus 20oC for a butane/propane mix.

The best solution to this drop-off in pressure/performance is to start with a canister which has been kept in a jacket or sleeping bag, or to gently warm the canister with a bowl of water or a gentle heat source such as a heat pad.

(Never let the canister become hot to touch!)