7800 plays 2600 Games, Not 7800 Games?

Hey guys, I'm expanding my Atari love from the Jaguar & picked up a 7800. It plays 2600 games fine but seems to get stuck on the boot screen of 7800 titles. Am I missing something easy here? Looked for switches on console, cleaned carts... no luck.

2600 Missile Command running:

7800 Ms Pac Man hung up on boot screen:

Thanks in advance for the help!

... also it feels like my 2600 Miner 2049er cart doesn't "fit." Just me being overly cautious?