Sometimes (especially later on in the chain of puzzles) when you enter the password and you're sure it's correct, the site will take you back to the top level page instead of to the next puzzle page. This is very, very annoying.
The best workaround I can give you is to hit Back, then reload the password page. If the password was correct, then the puzzle page will be unlocked and should display for you.
Honestly, I'm not sure what else I can do about this. I've experienced it myself during testing, but I can't figure out what's going wrong. Chances are that it's two or more of the plugins interacting with each other or the web server in a weird way.