Since scribe 1.3.0 there's a debug mode you can use to help yourself (and others) troubleshoot.
Usage is simple, just add the debug() call while you're creating the service like this:
OAuthService service = new ServiceBuilder()
.debug() // here!
Debug will output to System.out some interesting information that will help you find what's wrong.
Also, you don't necessary have to use the default output, since there's another method that accepts an OutputStream to send the debugging information, just in case. It's called debugStream and, as previously noted, takes an OutputStream of your choice.
These are useful weapons to fight the OAuth corner cases, I hope you find them as useful as I do. :)