When I send emails to a group I made in address book, a few people get the email but most don't. When one of the people who got the email hits reply all, everybody gets it. Why aren't my emails going through to everybody? Then they get it with reply all?

Welcome to Apple Support Communities.

It's probably quite simple, and nothing to do with your Address Book group.

It is a matter of 'junk mail' or 'spam' filtering on the recipient end, I think.

1. Apparently not everyone in your Address Book group (12 of 15 apparently?) has your current email address among their 'permitted email addresses', so, your original email message ends up in their 'junk mail' or 'spam filter' or whatever it is called by their email provider. Some businesses and schools also filter incoming messages for subject, language, content, or by sender's address (one company I dealt with prohibited all incoming email from Hotmail, for example), so it could be filtered at that level. In that case, the recipient might have to specifically ask the IT department to add your email address to a 'permitted' list.

2. When someone else in the group hits 'reply all', THEIR email address IS among the 'permitted email addresses' for everyone else, and they are in effect 'forwarding' your message from their email address, so the filtering affecting your email address does not affect their re-sending of your message.