That is probably happening because some filter category was left empty.
You have to be a super admin to perform the following. If not, contact the super admin of your company and ask her/him to perform it:
1. When in the onboardees' page, select "no filters" in the filter box:
2. You should be able to find the email;
3. Go to actions -> edit;
4. Check the filters and add the ones missing.