My API key works, but my client's doesnt

Hello Mailchimp Plug-in guru’s,

I’m building a website for a client that uses Mailchimp. I installed the plug-in so users can sign up to her newsletters and it worked great! That is, using my own API key.

When we try to input her’s, the plug-in says its not a valid API. We have tried regenerating it a few times, and reinstalling the plug-in. Its not working.

Any tips?