Hi
Check that you have removed all wrapping from the Magnetic Charging Dock and that you are using it only with a genuine Apple Lightning to USB Cable and Apple 5W USB Power Adapter - or Apple-certified (MFi) equivalent. Also try using a different Apple cable or 5W adapter (for example, as supplied with your iPhone) if you have them.
If the issue persists, contact Apple Support (mail-in service may be available) or make a Genius Bar reservation with a view to having your watch, dock and power accessories checked under warranty:
https://www.apple.com/support/watch/contact/