This issue is probably related to filters.
If you are an admin, go to the backend and follow the instructions below:
1. Go to user settings;
2. Click on onboardees;
3. Type the email of the person who is facing the issue;
4. Go to actions -> edit;
5. Check the filters and add the ones missing.
In case you are not able to find the person when typing the email you should try the next (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.