Okay, love my S21...just upgraded from the S9. My S9 would charge just fine anywhere. Plugged in, or wireless on a pad or wireless in my truck. My S21 charges plugged in with my fast charging travel adapter and on a pad with the same adapter. When I put it on the wireless pad in my truck, nothing. Like it's not even there. I've flipped it 180 degrees, slid it up and down and all around and still nothing. With a case, without a case. I was thinking that maybe my charger pad in my truck isn't putting enough watts out. I checked the setting on my phone and under More Battery Settings, Super Fast Charging was on and Fast wireless charging was on but Fast Charging was off. Do you think that might be it? I've turned that on and I'll see if that works, but other than that, I'm stumped.
If it charges fine wirelessly on the pad, then there is nothing wrong with the S21.
It could be that the built-in wireless pad in your truck isn't working. It also could be that it doesn't detect the S21 wireless coils for orientation, spacing, or some other reason.
The "Fast charging" setting is for wired only so it wouldn't affect wireless charging.
Did you check it without the case on the phone?
Galaxy S21 Ultra 512g(VZW), Galaxy Book (original, VZW), Tab S4, Galaxy Watch3 (VZW)