Some services are shown as "Offline" in the ISPConfig though they are running on the server.

The ISPConfig checks the status of the services on localhost (IP: 127.0.0.1). It may be that some of your services only allow connections to specific IP addresses and not 127.0.0.1 (in Apache this is done with the Listen directive). If you allow connections to localhost the status will be shown correctly in the ISPConfig.

Another possibility is that the affected services do not run on the standard port intended for it.