Membership Not Processing to

I saw one similar post about membership not processing but there was no posted resolution. Here's my dilemma:

Membership allows someone to register (create an account), select a membership (only one membership on this site), and fill in credit card details (with SSL) [to the page where it adds to the url "?action=registeruser&subscription=1"]. When the button "Process Payment" is clicked, the only thing that happens is that the page refreshes with all the inputs (name, address, etc.) still there. The information is not passed to at all.

I have been through the manual numerous times and searched Google for this, but can't see where the problem lies. I have deactivated all plugins but Membership and still have this problem.

I am using Membership Version, WordPress v. 3.5.1 and MarketPress theme. setting is in Sandbox mode. Everything is set up as described in the manual.