custom page template not showing up
Reason of page--membership.tpl.php not working for sitename.com/membership may be:
Drupal used templates as page--[front|internal/path].tpl.php based on the internal path of the current page. Do not confuse the internal path to path aliases (Pathauto module uses path aliases) which are not accounted for.

To create the template for page--membership.tpl.php one may try one of the following methods:


Create template as page--node--[NODE_ID].tpl.php, here replace the [NODE_ID] with the node id of membership page.
To get recognize the aliased paths, you have to add the aliased path as part of the theme suggestions: https://groups.drupal.org/node/130944#comment-425189