Why would it not change gears?

Hello,

Any time there is shifting issue the first item to check would be the transmission fluid. As low fluid can prevent shifting. It can also be due to a faulty shift solenoid or control module.
There are several different reasons for this to happen.

Typically, if the issue is module related it will trigger the check engine light.
Is your check engine light illuminated?
If so getting is scanned to retrieve the code(s) can help with the diagnostics.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

First make that the fluid is at correct level and in good condition. Dirty fluid can also cause shifting issue.

Here is a helpful guide on automatic transmission issues:

https://www.2carpros.com/articles/automatic-transmission-problems

Run thru this and let me know of any questions.
Thank you.