Apple Watch Battery Decreases While On Charger

I put my watch on its charger when it got down to 10% battery. It charged up to 88%, then the battery started going down. After half an hour, it’s down to 66%. I tried resetting it, that didn’t work. I tried putting it on airplane mode to decrease background activity, it’s still going down. Both my watch and phone are on the latest OS. Why can’t I charge my watch?