Through in a sheet to the developer without exposure and if it comes out black that means you're out $30. sorry. It's happened to me three too many times...

I use Kodak Supra Endura with the Kodak RT developer and it's currently working beautifully. I'm told it's more sensitive than black and white papers which is probably true so make sure your darkroom is light tight. And I would kill any safelights you dare be using until you're sure you've eliminated the problem.