This is brcause they are not perdectly alligned. Wireless charging is inefficient because a slight change in alignment can make it to just show the notification of charging while it is not. Do use wired charging because it is 47 percent more electricity and energy saving.
Your wireless charger coil becomes weak, try with some other Samsung charger. Also avoid back covers during the charge as only Samsung original or couple of 3rd party covers can do a proper job.
