Is it possible that the POST size exceeds what is set in the "Maximum size of post data" setting in CFAdmin (not likely as I think it defaults to 20MB or something like that)? Is the JSON being submitted using POST or GET (GET uses URL parameters and there is a limit to how big a GET request can be)? Or maybe the web server (IIS or Apache) is truncating the data?
Have you checked the JSON being transmitted via Developer Tools or a tool like FIddle or Charles? Is it being truncated before it even gets to ColdFusion?
-Carl V.