Authorize.net AIM Checkout Not Completing Process

I was having trouble adding getting this plugin to work for one client's site with PayPal Express Checkout. The thread for that is here: https://premium.wpmudev.org/forums/topic/whoops-looks-like-you-skipped-a-step-please-go-back-and-try-again-1

So I thought I would add it to another client's site who uses Authorize.net AIM. I tried it there and the same sort of thing happens, but a little different. I am using it Live. Now, after I enter my credit card information, I get taken back to my https page: mysite.com/store/shopping-cart/confirm-checkout/

and it gets stuck with a message at the top:

You are almost done! Please do a final review of your order to make sure everything is correct then click the "Confirm Payment" button.

I click on "Confirm Payment"button, and the exact same page reloads with the same message at the top.

Any suggestions? Since I am getting a similar result on two different payment gateways, on two different hosting servers (both LAMP), it might be something glaring that I am missing, or there might be a problem with the something in the plugin like the session handler.

Please advise.