Why does my return not change when I add my mother as a dependent?

Adding an adult as a dependent gets you the NON-refundable $500 credit for other dependents, which can reduce your tax owed.  It shows up on line 19 of your Form 1040.

CREDIT FOR OTHER DEPENDENTS

https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-credit

If your refund isn't changing maybe you are already getting back all that you can and adding more deductions won't change it.