Thanks for the heads up on this thread Raam.
I’m sorry for any confusion. However, actually, that is NOT the case.
s2Member Pro Billing Modification Forms do not depend on the Silent Post integration with Authorize.Net in this regard. You will want to have Silent Post enabled with your integration, so that s2Member can receive payment notifications from Authorize.Net, but upgrading from one Membership Level to another via Pro Forms is not dependent upon this aspect of your integration.
Perhaps you were testing the site in a way that restricted access to only certain IPs? Or anything like that? Perhaps you require a special cookie to access your site during development?
Ultimately, all transactions are processed together using s2Member’s core PayPal processors, which are an internal core functionality for s2Member. If this happens again, please take a look inside your logs directory, for anything that looks odd inside: /s2member-logs/ authnet-api.log or paypal-ipn.log. The Authorize.Net Pro Form processor will make a proxy connection to s2Member’s core IPN handler. If that fails due to some unknown reason, you might have issues.
If you run a transaction like this, and nothing comes into the core IPN handler (i.e. you have no paypal-ipn.log file), something is very wrong.