I made my own "Invasion"-Map and set up an attack where soldiers and 2 catapults should come.
The soldiers spawn, but not the catapults.
What am i doing wrong?

The AI can sometimes be very picky about where it wants to send its siege engineers and doesn't actually build the catapults you assign it. All you can do is try to change the layout and hope the AI likes it enough to build its catapults.