App Store keeps asking for the password even though it's correct?

When I type in my Apple ID password to purchase an app, it doesn't say that it's wrong, but it then says back on the app 'install' or 'free' again? It won't download no matter how many times I put in the password? By the way, I'm using an iPhone 4 iOS 7

Wow! This a great questions. I'm waiting for best answer.