Password Protection

I am testing WP File download with a folder structure consisting of two categories with each having sub categories. Each main category and their sub categories are password protected. I am having two issues.

I am testing on several computers and with different browsers.
- at times the password protected categories are visible with out entering a password.
- once a password has been entered the category remains open and visible even after clearing local and server cache. I have also configed the cache to not cache the page with no luck.

I just want the users to have to enter a password to access a category when visiting the page and then password protection reapplied in a defined time.

Any suggestions would be appreciated.