Take on the iPhone calendar

I'm getting stood up left and right and couldn't figure out why, until today. I've successfully used the iPhone calendar for years but ever since I got the latest iOS the functionality has gone to hell. I would always schedule an event and then add people to it using the "invite" function in the edit mode but recently whenever I do this, the person I'm inviting stands me up saying they never got my invite. It's odd because I am doing it the same way I have always done it and have never had any issue but recently I have nothing but headache.
If you look at the calendar event you will see a little "?" next to their name as oppose to the green check mark or red X if they have either accepted it declined the invite.
I looked in the iGeek online forums and they aren't much help as the best explain action I can find is that because people have an iCloud account with corresponding email address which they never use, the email invites just flyer lost in cyberspace