...
The first step to solving this issue is to understand which control(s) are causing this issue. The Firebug tool, http://getfirebug.com in the Firefox browser is a great way to find hidden controls which are invalid or required and causing the submit button to remain disabled. Test your form and enable the firebug tool. Open firebug's HTML tab and search for all instances of s-invalid using the search box at top-right corner. This will take you to the HTML of each control in the invalid state. The control's name can be found in that HTML. Now you can edit your form in the designer, find that control and fix the issue.
Note that if a required control is inside a hidden section, then that section will also be invalid and you will also see s-invalid in the section control's HTML. This s-invalid is not the root cause of your bug. You will have to continue searching with firebug further to get to the actual control inside the section control that is the root cause.
...