1. Check the filter settings for your calendar.

2. Ensure that all relevant events are visible in the calendar and have been associated with the Category in question.

3. Consider switching to another view such as day or week view.

4. Try clearing the calendar cache by going to File > Options > Advanced > Calendar > Clear Calendar Cache.

5. Check for any recent software or Outlook updates.

6. Contact Microsoft Support if you are still having issues after trying the troubleshooting steps.