Here's the story:

I am the author and copyright holder of some GPL software. Even though I don't think I'm losing money because of the violation, I feel a duty to stand up for the GPL, so I do want to follow through. I don't see instructions on how to do that here: http://www.gnu.org/licenses/gpl-violation.html

My GPL software's home page is here: http://ifdefined.com/bugtrackernet.html And a demo here: http://ifdefined.com/btnet/bugs.aspx

The company that I believe is violating my license is here: http://ibn-distribution.de/sbhd/

I have zero doubt that their software uses my codebase, but I'm not 100% sure that they are failing to open their source, as mandated by GPL. So, I contacted them directly using two different email addresses I found on their site, but I have not heard back. I wrote that I want to work with them to make sure they comply with GPL OR work with them to license my software under different terms. I don't care which, but as I said, I feel it as a duty to follow up.

What should I do next?