Hello Paige,
After clicking on the Invitation Email and then Send > Manually, you'll see the words, "Advanced Search" next the the magnifying glass. Here you can filter your search option and make sure that the first bullet is marked as "No" (Only include invitees who have not received this email in the last 6 months).
Another possible reason that the person isn't receiving the Email Invitation is due to the fact that the person you added isn't a valid email address or they are marked as "Opted-Out". You can find out by going to Contacts > finding the person in questions and click the name > scroll down to Opted-Out and make sure it says, "No".
I hope this helps...