Campaign not actually sending a particular subscriber email even though it says it has!

I’ve recently added a new subscriber/email to some lists and then requeued some recent campaigns that were sent out to those particular lists last week so that this new subscriber gets those messages.

The Campaigns tab on the subscriber’s dashboard shows that the message was sent out to all 3 lists (see attached screenshot where it shows it was sent to all 3 lists ) but one of the messages didn’t actually get sent! Have checked that the subscriber is actually on that list, which it is (I guess the message wouldn’t have shown up on the screenshot if it wasn’t).

Have now tried for a 2nd time 24hrs later and still no success. Can anyone think of a reason why this is happening?

Thanks

Phil