Hi!
Our robot is finished but we tried to use a catapult and the result was that the large balls got stuck with the intakes that we use to take them and the pneumatics don’t have the force to push the large ball far.
Can someone explain us or show us how we have to put the catapult so we can have a functioning catapult?
Seems like you have the same problem as me, I posted a thread: https://vexforum.com/showthread.php?p=352622#post352622
Posting pictures would be helpful, so we could help you better and look into the problem you are having with your catapult! 
Move the contact points of the pistons closer to your pivot point. Also if the large balls are having trouble clearing the lift, then you can build a small “stand” for the large ball to rest on. A great example is here: https://www.youtube.com/watch?v=MFdfHArsNZk
One thing I learned from experience today (after tweaking and testing for nearly a week) is that you need to hold down the button at least until the ball has been fired instead of tapping it as everyone I my team did. 