Samsung Galaxy S — WhatsApp Blue vs Grey Checks/Tick Question

What does it mean when I can see a contact's last login date and time, they have read many previous messages indicated by blue checks, but the most recent message only has 1 grey tick/check indicating its been sent but not received. I can see the person logged in a few days ago, pointing toward they not blocking me. Is the phone just simply turned off? Or is there another way to create the scenario of "last seen....", blue checks and 1 single grey check.

Yes I know these basics, but if I have chat where the other person was last seen 3 days ago all messages until then are blue checked, but the last 2 messages from 2 days ago are only 1 grey check. I can add this person to a group chat and I can see their "last seen..." so they have not blocked me. So why has the message not been delivered to the device.