@MM3301: Further to the advice by @Babishko you may wish to try heading to Settings > Battery and Device Care > More Battery Settings, and toggle on both Fast Charging and Super Fast Charging. If the issue persists, try toggling off Fast Charging before your next charge to see if this makes a difference. If it doesn't, can you confirm if you are using the phone while it's charging?
I have the same problem! Everything is on and not using the phone while doing it. It stays at slow charge. When I plug it in it says charging - 2.5hrs and switch to 3.5hrs. I tried the original Samsung plug and 3rd party. Same results
Same thing is happening to me after going with my S21 Ultra in the sea. Moisture notification everyday and Fast/Super Fast charging not working anymore (after one month since it had the water contact). Meanwhile i have cleaned the charging port with alcohol, tried with 5 different cables, same issue even after factory reset. With Galaxy S10 this never happened after hundreds of hours of being in water.
@Samsung, it looks like you have a real problem with this product. 1500 euros have an annoying and slow charging phone?
Hello one cause for the issue is humidity level and a second cause is heat both processor and battery heat can cause the feature to be disabled. I would recommend never leaving the case on the phone if you are trying to fast super fast charge. Also do not have case on while using the phone if you are planning to fast charge at the same time. It is also important to ensure that the power brick and phone is not in contact with soft surfaces while fast or super fast charging. Heat lowers the power bricks efficiency leading to less usable watts. The phone will slow chatting to protect the battery which can lead to evan slower charging speeds. If the issue persists try placing the phone on a two pencils and continuously blowing cool air under and over the phone. This helps maximize heat dissipation. If this solves the issue than you should probably make sure the phone is completely turned off when you wish to super fast charge.