I have an S9+ and the samsung wireless charger. The blue light blinks indicating that it is not properly charging and the phone won't get any juice with the screenless Otterbox defender case - I just removed the outer layer of the otterbox and the phone is now charging. But it is going to be a real pain in the butt having to do that every single time I want to charge my phone wirelessly. #1stWorldProblems
I have the same case and I just got the S9+ today, I came here not being able to get my phone to charge, but after following the directions from brockmoel I can definitely say that I can get my phone to charge without removing the case, but it's a bit fiddly to find the sweet spot, but then once it starts charging it's fine.
I have found that the only way I can get the phone to charge properly is if the wireless pad is flat on the surface, if it's raised at 90 degrees I can't get it to charge.
As with what someone said about the S8, the sweet spot seems to be to have the phone sitting slightly off the vertical centre, if the middle of the phone is sitting in the middle of the pad I end up with the light initially coming on, but then going off, but if I push the phone up slightly the blue light on the charger then stays on and my phone charges.