I'm just a student doing research, trying to use their code. I don't even want them to fix the code, I just wanted to ask the primary sources about how they implemented a few things. I don't have to ask them, I could go to someone else or a forum and ask the same thing. Maybe I'm naive, but it doesn't take a whole lot of effort to just respond back and tell me that they don't have time.
Try to make your company offer them money to cooperate. They might be suddenly very interested in your questions.