How would you explain a person who answer the form/survey from the link in any email, but NOT registered as click in the report, and has not been marked as invalid or suspended?
Thank you all for the help.
First I'd check to make sure the click was tracked for anyone (i.e. if you embed the link the wrong way, it won't be rewritten to bounce off the tracking server).
If it was tracked, then look for someone who might have forwarded the link or — more disturbing, but it happens! — put it on social media.