PROBLEM TO SEND THE CANDIDATE ADDITIONAL TESTS
If this is a problem that is related only to one candidate and all the other ones works just fine, check the way the email address of the candidate is written. In one cause there was a space before and after the email address.

Delete those and the additional tests will be sent to candidate.
Go to the "details of the candidate" and see if it does not need to be corrected also there as then you might have a problem to send to this candidate also general emails from the system.