IAX2 codec negotiation failure

I have a setup with 3 Asterisk sites which are connected using IAX2 and inter-operating OK. I’m now adding a 4th site, and IAX2 registers OK with the other 3 sites, but when I attempt to make a call to or from it, I get a message simply saying “Unable to negotiate codec”. I’ve checked that they are attempting to use the same set of codecs. I’ve tried setting high numbers (up to 10) for debug and verbosity levels in the CLI, but the error message is no more detailed. Is there any way to view the details of the negotiation process to help diagnose the problem? (The problem server is running Asterisk on Fedora 14 32-bit).