What version of Zen Cart? What version(s) has it been upgraded from? When?
What addons do you have installed?
Which Authorize.net module are you using?
Have you tried enabling the debug-log-to-file option in your Authorize.net payment module settings? The log files will be put in your /cache/ folder. What do they contain ... they should explain what happened in the transactions.
Did the customer see any error messages?




