When troubleshooting this I would make sure that your Apple Watch charger isn't damaged in anyway, and if you have multiple chargers try using another one. If your watch hasn't been charged in awhile sometimes it can take a little time for it to have enough charge to give you an indication that it is charging. Here are some other steps you can try to troubleshoot the issue:
If your Apple Watch won't charge or you see a message on your watch that charging with this accessory isn't supported:
- Make sure that you plug the Apple Magnetic Charging Cable or USB-C Magnetic Fast Charging Cable all the way into the USB Power Adapter, then make sure that you plug the adapter all the way into a power outlet.
- Make sure that you're using the Apple Magnetic Charging Cable or USB-C Magnetic Fast Charging Cable that came with your Apple Watch.
- Completely remove any plastic wrap from both sides of the charger.
- Make sure that the back of your Apple Watch and the magnetic charger are clean.
- Position the back of your Apple Watch on the charger. The charger's magnets align your Apple Watch, and you should see a green lightning bolt on the watch face.
- If your Apple Watch still won't charge, force it to restart. Press and hold both the side button and Digital Crown for at least 10 seconds, or until you see the Apple logo.
- Try a different Apple Watch Magnetic Charging Cable or USB-C Magnetic Fast Charging Cable, and a different USB Power Adapter. If your Apple Watch is out of power, you might need to wait a few minutes for the green lightning bolt to appear after you start charging your device.
If these steps do not help you can also contact Apple Support.