I'll try to give all the details as best as possible:
- As a side note, I have had an account since '08, but was inactive for several years. I last messaged someone on the old system 4/5 years ago. I log in to find the new messenger system put in place. AFAIK on the desktop website the new messenger didn't affect how you send messages, other than the notification doesn't seem to be showing sometimes.
- I sent a message to someone I'm friends with using the website through a desktop browser.
- The friend does not have the messenger app on the phone (or least it says they don't when I installed the app on my phone). I had installed it after sending the original message, but wouldn't think this has anything to do with it.
- I've seen the friend be "active" according to FB and have a something like "1h ago" when I check if they responded on the messenger app I installed on my phone.
- It's been over a week and the message was still showing the "Sent" icon (Non-filled in check mark), but never shows the "Delivered" icon (Filled in check mark)
- I've sent another message which also didn't get delivered (I've stopped at this point, because I figure if the friend ever receives them they will get spammed with a bunch if I send any more).
- I have sent messages both on the desktop website and on messenger to both old friends (like the one above) and new friends I've added since with no issue. They are noted on messenger as sent, delivered, and seen with no issue and show a check mark on the desktop website with "seen".
- I'm not blocked by the friend otherwise it should have given me a message.
- I don't feel the friend would be using a "message read" masking app considering they don't even have messenger installed.
Is there something I'm missing or does this sound like a problem on FB's end?