Apologies for replying to this old thread but I stumbled on it due to having the exact same problem. I was working with a fresh install of LB and RA and just followed the LB tutorial for adding RA to everything. The first platform I brought over was the original GameBoy. I had downloaded the required cores on RA and set up full screen mode but the games wouldn't launch. I pulled over a GameBoy Color rom and that started without issue. With following this thread and the fact that the color game worked I took a closer look at the associated platforms on the RA emulator and noticed that it only had the GameBoy Color listed but not the GameBoy. I added the GameBoy at the bottom of the list, associated the necessary core and bingo.
I hope this helps someone in future cause it wasn't an obvious issue for me.
