Sounds like Authorize.net is having trouble confirming that your customer is being properly returned back to your website.
ie: when their page sends the customer back to your site, they wait to get a response from your server ... but they're not getting it.
So, either your website is very slow, or an error is occurring.
Maybe you need to do a checkout for yourself to determine what's going on. Or ask your customers.
Or, better yet, use the AIM module instead ... which doesn't have to rely on that whole process that SIM is having problems with.
https://www.zen-cart.com/tutorials/index.php?article=373
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