One of those forehead slap moments. I was trying to think about what might be different between the setups (except for about 3 years of RetroPie development) when I realised the cable/wifi thing. I'm glad I worked it out, because my next option was to undo the NAS and put everything on the micro-sd, but the one I'm using isn't big enough, so I'd have to get a bigger one and go through the hassle of moving everything over, etc. Glad I don't have to do that.