Password protect page not working

I’m trying to password protect a single static page on a client’s site. I did the normal run of setting up the page to be password protected, but after entering the password the first time I haven’t been prompted to enter the password again, on any browser or computer.

This is what i done so far:

• Cleared cache, and active logins.
• logged out of the admin
• Turned off and removed all plugins, re-installed them
• Set permalinks to default, set them back to /%postname%/
• entered if(!post_password_required()) { the_content();} on the template that is supporting the page
• I didn’t update to the latest version of wordpress because it all ready is

So having done all that I know the problem is the theme, but my client really likes this theme so switching to something else isn’t really an option and is the only reason I haven’t done it.

If someone can suggest a hack or plugin I can use to fix this, I would really be thankful for it.

Any advice please?