I sent an invite (my last!) to a friend a couple days ago to a gmail address, and he still has not received it, and has checked his spam folder as recently as a few minutes ago. Is this a known problem or is he really doing something wrong?

I have sent 3 invites previously that were successful; this is the first one that is not being received.

Can I rescind this invitation somehow so I have it to send out again, either to another address for this friend, or to someone else?

Thank you!