Follow the steps below to troubleshoot and fix OneNote highlighter not working properly:

1. Check the batteries. Replace with a new set of batteries if necessary.
2. Check if the On/Off button has been switched to On.
3. Verify if the ink cartridge of the highlighter has been refilled properly.
4. Clean the pen tip if it is clogged with dried ink or other debris.
5. Check in the settings if the On/Off power level also has been switched to high.
6. Reset the highlighter by unifying the pen.
7. Test the highlighter’s performance on another device if necessary.
8. Install the latest updates for OneNote.
9. Ensure that the OneNote application is up to date.