make sure to check your settings for activating wireless charging in the first place! but also remember a lot of older wireless or 3rd party pads/stands may not work with our new models.
lwireless charging is by no means poor, its a luxury and like most things that category it isn't perfect.
however, with your S21/+/U I would invest in a 15 to 25w Official not 3rd party, or at the very least a pad or stand from a known and trusted brand and use it with a original Samsung power brick at the same wattage alongside your phones original USB C to C cable.
I have that setup and not only does it deliver 40% of the voltage it is able to it also cause zero damage to my device , it helps maintain the battery by not using the fast charging options unless needed (cable or wireless).
fast charging , super fast and charging over night as well past 80% multiple times a day is your 🔋s worst nightmare