There can be several reasons for this: Your Polar H10 is connected to both iPhone and the Apple Watch but you haven't enabled the Dual sharing mode. The recent firmware update on the Apple Watch or chest strap may have caused the issue. The battery life of the Strap is coming to the end, or low at the time of connection. This issue is usually resolved by unpairing and re-pairing the devices. If that doesn't work, try factory resetting your Polar H10.