The Saturn cartridge slot can be very temperamental. The things I would try, in this order:

• Push the cartridge in all the way and then back it out just 1mm and see if that makes better contact
• Put a thin card behind the cartridge so it is pushed forward just slightly and see if that makes better contact
• Disassemble the console and use some contact cleaner on the slot. Be careful not to bend the pins.
• Loosen the screws that hold in the connector just slightly

When you find the right position to get a cart working, it’s best to leave it inserted. This is why many people use the multi-functions carts, so that they don’t have to constantly swap carts and go through the process of trying to get a good contact every time.