Why do some iMessages say "delivered" and some dont

This is kind of a funky feature within iOS. If it doesn't say delivered then iMessage may be having some connectivity issues through your phone. What you can do is hold down the message and select "send as sms text message". If you don't have this enabled you can do to settings --> Messages --> and toggle "send as sms". This will send your text message when iMessage is having issues.

Hope this helps.