Paid Subscription issue
 

I have recently set up a test subscription on a VBulletin forum, registered users have "read" rights but no posting abilities. Only subscribers have "post" rights. All of this is working correctly.

The paid subscription is set to assign users to a group after the payment, and is set to use Authorize.Net as the payment API.

I ran a test subscription with one of my accounts and the payment was processed, shows in the transaction logs, and my new group and rights work correctly. The subscription panel shows a new subscription with expiration set for 14 days. So everything worked as intended.

However, this is the problem:
Every other subscription has not worked. I have tested the subscription and payment processing 5 more times now, from the exact same machine / browser; as well as 3 other machines and different browsers (Mac, PC, IE, Firefox) and something isn't working.

I received the Email receipt from Authorize.Net, and I was redirected to the completed payment page and the subscription user panel. But no subscription is displayed.

I checked the transaction logs and nothing is there: No errors, no confirmations, nothing. The initial test I did shows in the transaction log, but no other transaction does.

Outside the system, from the Authorize.Net site, all the transactions are there and look to be billed correctly.

My merchant account is still in test mode, so I've been testing with the standard test credit card numbers, I've tested it with different ones and they all exhibit the same issues (payment succeeds, nothing is logged and no subscription is set).

Are there any known issues or limitations that could be affecting this?

Any help or pointers for where to look would be greatly appreciated.