Please excuse if this sounds like a newb answer, but ... which payment modules are enabled in your admin?
Are these orders being placed via the "Credit Card" module? and not actually the Authorize.net module?
Maybe they're never hitting authorize.net because you're not actually using the authorize.net module?


Somehow I don't think you're even using the authorize.net module. However, if you need to audit what the module is doing, go into the authorize.net module's settings and turn on the debug logging. I think it stores its logs in your /cache/ folder, so make sure the folder is writable. Or just use email mode instead of file. THen do another transaction. THe logs should tell you what it's doing, including the URL it's posting to. That should tickle your authorize.net tech folks nicely. Granted, if you're not actually using the authorize.net module, there will be no log created. The couple times I needed it, the log was helpful. Turn off the logging when you're done though.