iMessage does not show the message was "delivered" at it should be on new iPhone 6S

iMessage does not show the message was "delivered" at it should be on new iPhone 6S
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#1

I just bought a new iPhone 6S yesterday. My mom and me text to each others through iMessage. However, after I sent messages, the messages do go go to “delivered” status even my mom received all already. What is this kind of issue?
Thank for your helps.


#3

@Jan: Just go to Setting —> Messages —> Turn Read Receipts On