Series 9 watch won’t charge fully
Despite turning off the optimized charge limit, my watch only charges to 81%.
Apple Watch Series 9
Question marked as
The first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
Other than that, your best option may be to make an appointment at the Genius Bar in an Apple Store near you and let them look at it. If that is not possible, use then online Apple Support option. Either way:
Genius Bar Reservation and Apple Support Options - Apple
They have advanced diagnostics they can run to determine if it is a software or hardware problem.