Within NaturalHR it is possible to enable and use modules while still hiding them from view under Self-Service, this means that only users with access to modules directly will be able to access them if this is the case. To enable this navigate into:

Administration -> Company -> Settings -> Display Settings

Then locate the 'Do you want to display different modules via self service?' and select Yes:

Once set to yes you can then specify which modules are accessible via self-service using the checkboxes under the 'Self-Service Modules' field:

If you have an instance where an Admin or HR user is able to see a certain page or item on the system, for example a performance review form but an employee cannot then it's worth checking the self-service modules area to see if the corresponding module is ticked and available, if it isn't then this is likely why the module isn't visible to the user and you can resolve this by ticking it and clicking Submit.