The wiki has two levels of permissions for wiki spaces:
- Space level
- Page level
If you can navigate to a wiki space, you have permission to view that space. If you can edit a lot of the pages, you probably have editing rights as well.
If you know there is another page, but you cannot see it, it probably has a viewing restriction (page level). A page can be set to be viewed by only a handful of individuals, so only they can see it. This page and any child page underneath it, will be invisible to anyone not on the viewing list -- think of this as a member's only club.
If you believe you should see a page but cannot, ask the space administrator to add you to the page restrictions (under Tools > Restrictions).