Pay by Credit Card using Authorize_net skips payment details step

An authenticated user is logged into their account. When the Checkout button is clicked in the Shopping Cart, and then "Pay by Credit Card" is selected on the Order Summary and "Continue with payment details" is clicked, it goes straight to "Your shopping cart Your order has been submitted. Your shopping cart is empty. You may continue shopping from our product page." No credit card information was collected.

The order appears as payment pending.

We have a valid authorize_net Login ID and Transaction Key set up with the https://secure.authorize.net/gateway/transact.dll url. PayPal works fine.

We don't know where to look to impact this situation.