About 10% of the time anyone with iPhone texting me results in notification that I have a mms message sent to me but it doesn't download/open. Sometimes it opens minutes/hours later, or not at all! I am using default Messaging. This is BS. 25 years into cell phone technology and iPhone hasn't decided to play nice with androids? I like Samsung and am more inclined to never buy an iPhone because they Could create text operating system that works flawlessly w/ Android, but choose not to.
It's because iPhone uses a different type of SMS, It can check if the SMS has been opened like an MMS Read Report, Android uses the normal SMS rules that don't have read reports, only delivery reports.
When the SMS is successfully received they're seeing that it's been successfully delivered, if you open the message, it'll say that it's been seen, like Facebook messenger in a way.
The OS's use completely different methods of SMS and probably will not become compatible anytime soon unless Android finds a method to handle the changes.