Having softmodded two Wiis for the first time yesterday, and having nothing but a stack of DVD+Rs, I can say with a reasonable degree of certainty that your discs aren't coasters quite yet. True, the Wii doesn't really like them very much (first three burns ended up coasters), but I did find a solution: burn using ImgBurn, and change the
book type to DVD-ROM.
I've burned a total of seven CDs like this, and aside from the first three where I didn't change the book type, they work fine. On an NTSC console running 2.2U with Gamma launcher, I didn't have to change any settings whatsoever (nothing forced), and ran two different PAL rips, one NTSC, and one scrubbed JP game. No problems at all. Of course YMMV, but the same setup (replicated the steps verbatim) on my sister's Wii also worked.
To change your book type:
Click the book button in the bottom-right.
In the window that comes up, find your drive's manufacturer. I'm not sure how I knew that my Sony drive was actually made by LITE-ON, but yeah, I remember seeing the company name, and the setting worked. /shrug
Change for the next write, but you may also want to change for DVD+R media. For safety's sake (well, and my wallet's), I set Next Write every time I burn a CD.
This is what you're setting the book type to - DVD-ROM. Select it, hit Change, and you should get a little success dialog.
Success! Hopefully.
Burn away (lots of people say to use very slow writing speed, but all three coasters were burned at 2x, and from then on I used max (for my drive, 8x) and all was well) and it should (hopefully) work fine. Hope this works, but, as always, YMMV.