Cold weather thickens your e-liquid, which means it doesn't flow into the wick as easily. Less juice reaches the coil, so you get weaker vapor and a muted taste. Warm the device in your hands or an inner pocket for 5–10 minutes, give it a few gentle dry pulls (without pressing the button), and store it upright. If the flavor stays flat after that, the coil may be burnt—time to retire the device.