imessages and read receipts
Other than they didn't actually open it yet - reading on the lock screen doesn't tag it as read - most likely is they switched off Read Receipts for some paranoid reason.



Either that or you're blocked - there's no way to tell that, unless you ring them &amp; get bounced straight to voicemail every time… though their phone could be off.  

Blocked messages are sent &amp; arrive at the receiver's phone, but are just never shown to them, so as far as the system &amp; carrier is concerned, everything went as planned.
Voice calls might ring once, then go to voicemail; depends how quickly the phone reacts to the block. Again, as far as the system &amp; carrier is concerned, everything happened as it should.

Last option, they sold their iPhone, bought a droid, &amp; forgot to switch off iMessage. [long shot]