Gift Certificate can't be used during checkout

I'm having a problem with my gift certificates. They can be redeemed but if a customer tries to use it during check out it doesn't work. If a number is entered into the apply amount box, it doesn't appy to the order. It just doesn't do anything and the order continues as if it wasn't used.

Does anyone know what the problem might be, or at least which files control this so I can know where to begin looking for the problem?

Also, I forgot to mention that customers can't send gift certificates either. If they try they get error messages saying the name, email address, and amount are all not correct.

Edit: sorry, I somehow pressed the wrong button. Please delete this post.

What version Zen Cart?

Clean install or an upgrade?

How did you install it?

How did you upgrade it?

What happens if you switch to Classic Template?