Applicant Tracking Software (ATS) systems are far from perfect. Sometimes we even know there are candidates in our database with the qualifications we need, but we can't find them when we search!
Unless you have a developer who can customize your ATS, it's going to take some extra work.
What can you do to get better results?
1. Use a wider range of search terms
2. Examine resumes closely to refine your search terms