Have you got the required transformations installed as detailed in the documentation.
Not all features work when using the home assistant mqtt standard.
To add them manually without transformations you will need to edit your zigbee2mqtt configuration yaml. This can be done in frontend if you have given it a port.
If you already have things setup using the home assistant standards set the output in experimental settings to attribute_and_json or add it via configuration.yaml.
experimental:
output: ttribute_and_json
That way you can add them manually (generic) or via the home assistant auto discovery) best of both worlds 