FormAPI callServer does not execute?

System

I have put debug statements in my code around a callServer call and I can see that the callServer is being hit and no error is resulting. But in access_log I don't see this CGI request. It works for one DCR but not for another DCR, so I assume it is some kind of data condition (something invalid in the parameters object).

Does anyone know what could cause this? Are there values that are invalid in the parameters object to callServer?