
When someone is talking voice, and out of a WiFi area, they may not be able to receive text messages, because they may drop or lose some of the data, so it is conveniently held onto for them until they're freed up (i.e. It also happens frequently if they are talking on the phone. This happens when a phone is off or out of service range, obviously. The most likely, and also the only I've confirmed, is that the recipient cannot receive the data. "Delivered" may not appear for several reasons. In some cases, the Messages app does not even show the recipient's name, only their phone number – even though these contacts are synced in iCloud.Basically, your message has been sent, from your end, to another iPhone or Apple apparatus (it appears in Blue and not Green) but has not been delivered to your recipient. This issue is only occurring with some contacts, all of whom are iMessage users. In the Messages app on macOS, I can receive replies from the other person, and I can see the messages I've sent to them via iPhone, but any attempt to reply to them via the Messages app in macOS is met with a "Not Delivered" error. Text message forwarding to my MacBook Pro is turned on in my iPhone's Messages settings. I have logged in and out of iMessage on both devices multiple times to no avail.

My iPhone is updated to iOS 14.3 and my MacBook Pro is updated to macOS 11.1. In each case, the person on the other end is using an iPhone and are logged into iMessage on their device. The issue is only occurring in macOS – I can send and receive iMessages and SMS in iOS just fine.

Ever since I updated to macOS Big Sur in November, there are certain contacts that I cannot reply to from the Messages app.
