According to support documentation updated today, Apple says the Apple Watch Ultra, larger models of the Apple Watch will need to be repositioned or laid completely flat when charging with the MagSafe Duo charger to ensure the charger is properly connected to the Apple Watch.
If the above still does not consistently charge the Apple Watch Ultra via the MagSafe Duo charger, Apple recommends trying to remove the watch’s band before charging.
As Bloomberg’s Mark Gurman pointed out last week, the Apple Watch Ultra is not included in the list of devices compatible with the MagSafe Duo charger, likely because its 49mm case size and bulky design make it unsuitable. Instead, the new Apple Watch Series 8 is listed as compatible, as are all Apple Watch SE models.
The MagSafe Duo, priced at $129, is a foldable mat with a MagSafe charger for iPhone and an Apple Watch magnetic charging tray. Apple released the accessory in late 2020 ahead of the 45mm and 49mm Apple Watch models. Note that the MagSafe Duo charger does not fast charge the Apple Watch Ultra, Series 8 or Series 7.