Hello rvenne,
Thanks for asking the Apple Support Communities for help with using your Function keys.
Please check in System Preferences > Keyboard > Keyboard tab > there is a checkbox "Use F1, F2, etc. keys as standard function keys".
Make sure this checkbox is not checked. This article talks about this: How to use the function keys on your Mac - Apple Support
If this box is currently not checked and it is still not functioning as described, try the following:
1. Check the box: "Use F1, F2, etc. keys as standard function keys".
2. Restart your computer: How to turn your Mac on or off - Apple Support
3. Once restarted, log in and unchecked the "Use F1, F2, etc. keys as standard function keys" again. Re-test.
Thanks and have a wonderful day.