Well first thing is have you tried using the S8 on another wireless charger? Now assuming you have (or you do and it works) I will say that is a very odd situation. And if the charger is at fault, there is really no fix for it that I can think of (call 1-800-SAMSUNG if you're in the US or your regional equivalent if you think there is still a warranty on ).
The only thing I can think of is the S8 does fast wireless charging, and the S6 does not (at least I think it doesn't, if it does, ignore everything I say below, I got nothing for you). Maybe the device can't do fast charging anymore and for some reason it can't work on the S8 because of that? Doesn't make sense but that is the only difference between the two phones. Try plugging the charger into a non-Samsung charger (so it can't do fast wireless charging) and see if the S8 works when plugged in that way. Now that being said, there is no fix I can think of for you if the wireless charger is the issue.