Make sure that your custom organization service implementation is fully compliant with Organization Service TCK tests. Tests are available as maven artifact:
groupId - org.exoplatform.core
artifactId - exo.core.component.organization.tests
You can find TCK tests package source code here
In order to be able to run unit tests you may need to configure the following maven plugins:
Check pom.xml file to find out one of the ways to configure maven project object model. More detailed description you can find in the dedicated section called "Organization Service TCK tests configuration"