I just tried several different Atari carts: Robotron, QIX, Donkey Kong etc and they all came up. When I plug the SUBCART in it goes straight to the Fujinet and acts like there is no cart in the system.
I tried several different games. ATR, XEX etc. I currently have no cables connected to the cart. I will get my other 800XL out later and try it on that. Never had an issue with it before today. Maybe I will switch out the SDCARD.