I had the same problem with my brand new wireless charger pictured like yours. I have an Otterbox cover as well on my S8 plus . It worked (fully charging) twice then all of the sudden it would stop charging at 28%. I'd wake in the morning with my phone charged only to 28%....never fully charged.
I tried every suggested fix I could find online, then decided to change the original plug that came with the wireless charger with a samsung spare brick that I had. I used the same original cord that came with the wireless charger. SEE PHOTO below
It works perfectly now. It's possible I guess that those folks experiencing problems like this may have faulty originally issued plugs. You might want to try as an experiment using the brick that was issued with your phone instead just to see if things work out. GOOD LUCK!!
with samsung phone charging plug
... View more