Help please with Slave flash on NX10

Has anyone used an optically fired slave flash with the NX10. I’m getting rather odd results.

I get a brighter photo with just the pop-up flash than if I add the slave flash. This raises two questions, one is simply how do I get it to work. The second is; how can adding extra light give me a dimmer photo, when I have fixed the exposure settings in manual. Even if the slave flash wasn’t in sync, surely the photo should be as bright as just using the pop-up.

Settings; manual exposure, 1/60, f8, iso100