Compost bin not breaking down properly

I started a compost bin about eight months ago and when I checked on it recently it's still mostly just a wet soggy mess rather than anything resembling compost. I've been putting in kitchen scraps, grass clippings and cardboard.
Is there something I'm doing wrong? I've read about the green and brown balance but I'm not sure I've been getting the ratio right. Any tips on how to get it working properly?