iMessage doesn't show "delivered" on sent texts. How come?

Hi everyone -

I sent a message to one of my friends who I regularly text. It's been a few weeks, and I hadn't texted this person. Then last night, I went to text them - it was a few pictures and some text. This morning I woke up and noticed the text sent as an iMessage (blue bubble) but did not say "delivered" on the bottom of the message, as iMessage usually does.

I've never seen this before, as everyone else that I text using iMessage, my messages go through as "delivered."

I've done a little Google research and am really unable to find anything on the topic, other than it could be that the person has their phone shut on/off. Well, this friend of mine is GLUED to their phone, and never has it off, and even if it was off, they would have received it by now, since it has been 12 hours.... It still does not say anything.

Does anyone know why this might be happening? Did this person block me? (I thought when you're blocked, it still goes through as "delivered" so you don't know). Did they really get my text, and iMessage just doesn't say "delivered" for some reason? (They are on Sprint, I am on Verizon... we both have iPhone 6+s .... not sure if that matters)

Any help anyone can give me would be appreciated.