Hi Nicole,
This happens, because the missing results does not contain that keyword. From the plugin perspective the two keywords are not interpreted as the same word in singular/plural – but two completely different words. For that, a much more advanced linguistics engine would be required, which is out of scope of this cheap plugin.
You can try an automated solution, which includes custom code snippet. Try adding this code to the functions.php file in your theme/child theme directory – make sure to have a full server back-up first for safety. For more details you can check the safe coding guidelines.
Then, you also need to enable the index table engine and configured it. Once it is set up and enabled, the custom code will force singular/plurar conversions and index those as well.