Pages Missing From Specific Post/Page Button Generator
You're trying to use s2Member's Specific Post/Page Access Restrictions, and although you've protected several Posts/Pages this way, they're not showing up in the Specific Post/Page Button Generator?
I would suspect that you've created a conflict by mistake. s2Member will not allow you to generate a Payment Button that facilitates Specific Post/Page Access, with any Post/Page which is already protected with Membership Level Restrictions (of any kind). WP Admin -> s2Member -> Restriction Options -> Specific Post/Page
Also, please be careful not to create a conflict with other Access Restrictions. If you are going to sell Specific Post/Page Access, you should enter specific Post/Page IDs here; and make SURE that you've NOT already protected any of these Posts/Pages with Member Level Access Restrictions.
In other words, if you configure s2Member, in such as a way, that a Post/Page requires Membership Level Access, you cannot sell that same Post/Page through Specific Post/Page Access. Doing so, would create a conflict. Customers that purchased Specific Post/Page Access, would be unable to access the Post/Page – without also having a Membership. Not good. So please be careful.
In other words, if the Posts/Pages are not showing up in the Button Generator, it's most likely because those Posts/Pages are already protected by s2Member's Membership functionality in some way.
I would review all the WP Admin -> s2Member -> Restriction Options to be sure these Posts/Pages (i.e. the ones that are NOT showing up) are only protected with Specific Post/Page Restrictions, and nothing else. Look for anything which includes multiple areas of your site, like Tag Restrictions, Category Restrictions, URI Restrictions, or the special keyword "all".