That suggests that something is breaking the process which occurs after the payment is accepted.
Have you tried a checkout yourself to see what exactly is actually happening? When was the last time you went thru checkout yourself?
What's showing in your site myDebug-xxxxx.log files?
https://www.zen-cart.com/tutorials/index.php?article=82
Which authnet module are you using?
When did this start happening? How does that coincide with ANYTHING related to your site? addons? upgrades? changes in your admin by any staff? etc
What version of Zen Cart? PHP? MySQL?
Who's your host.
And any other questions mentioned in the "Posting Tips" section of the screen after you click the Reply button below.