My new 2DS XL won't accept the old DS games, do I have to pay attention to anything?

I've just bought a new New Nintendo 2DS XL and spent extra days researching whether you can play the old DS games on it. The answer was yes, now I wanted to play an old game and he doesn't read the card. I've tried all the games on my Nintendo DS Lite and the 2DS XL doesn't read them, but it reads my new 3DS game. All of my games work fine on the DS Lite apart from the occasional reading problem when switching games.

Do I have to pay attention to anything, change something in the settings, does anyone happen to know anything about it?

Please reply quickly, I have a deadline to return the console.

It should actually read them just like the 3DS games. It can't really be because of the settings.