Hi @Ben12345 Have you being using compatible 25 watt Samsung chargers such as these from the Samsung store https://www.samsung.com/uk/mobile-accessories/wall-charger-for-super-fast-charging-25w-black-ep-ta80... https://www.samsung.com/uk/mobile-accessories/wall-charger-super-fast-charging-25w-ep-ta800xbeggb/ You should have got USB-C cable with S21 Ultra though. Do you have Fast charging and superfast charging turned on, see guide here https://www.samsung.com/uk/support/mobile-devices/how-to-use-and-charge-galaxy-s21-series/
can you check if there is any debris in the charger in case that is the issue. if still no resolution to charge in safe Mode and/or an idea to wipe the cache partition:
Now, you should see Safe mode written at the bottom left corner of the screen. If you see your phone charging in safe mode, it means some third-party app was the trouble maker. If not, we recommend you move ahead with other methods.
Note: To exit Safe mode, simply restart the device.
Wipe cache partition:
Press the Power key to restart the device
see some more information here https://www.techietech.tech/how-to-fix-galaxy-s21-series-not-charging-problems/
know of someone who had trouble with charging whilst on the Beta and this solved the issue, not sure if you are on one Ui 4.0 Android 12 as stable version as rolled out on the S21 series.
However if no resolution would be best to contact your provider or contact Samsung support as in that case the device may well be faulty.
Got my S21 Ultra about 2 weeks ago and suddenly my phone won't charge when I plug it in to the super fast charger. So I plugged it into my old adaptive fast charger and it worked perfectly fine. Any thoughts on how to fix that. ?