Not Keeping the Software Updated
Like all of Apple’s devices, Apple Watches receive periodic watchOS updates that contain new features, security patches and other under-the-hood changes. We recommend installing these as they become available.
To do so, just open the Watch app on your iPhone and navigate to General > Software Update. Make sure to keep your Watch on its charger and at 50 percent battery life. While the Apple Watch is updating, keep your iPhone nearby.