Apple Watch ultra charging issue

If a force restart does not solve the issue, as it did with my Ultra 2, and you can not try a different 20W power adapter, you should contact the official Apple Support about that. The can diagnose your watch online, if needed.

Contact Apple Support - Apple Support

Before that, and to rule out a software issue, erase the watch and pair it again, setting it up as a new device. You will only lose your created Watch faces and may have to activate the cards in your Wallet app again.

Then test the fast charging again.

Unpair and erase your Apple Watch - Apple Support

I have a similar problem with my Apple Watch Ultra 2.

I was able to fast charge my Apple Watch Ultra for more than a year. Since about a couple of weeks, it is no longer the case. I have restarted my watch. I use the same chargers and the original charging puck. Nothing has changed. Now, it takes about three and more hours to get about 80%. I have the latest IOS and the latest Apple Watch OS.

Are you using at least a 20W Apple power adapter and the USB charging cable which came with the Apple Watch?

Did you try to force restart your watch?

To force restart your watch, press and hold the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again.

If your Apple Watch won't turn on or won't charge - Apple Support