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

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.

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