Contact Form plugin - Not receiving emails at separate addresses
First, make sure you're on the latest version of the plugin. There was a bug related to sending to multiple addresses that got fixed several months ago.

If that's not it, have them check their SPAM folder to see if it's getting trapped in there.

If it's not in SPAM, then try using a different email protocol under Settings->Email in the control panel, preferably SMTP based.  phpmail is notoriously unreliable at email delivery.