How do you get the Grappendix to grab onto a projectile?

[Captain Facepalm]

I got it to work a few times. Now, I can't get it to work. What am I doing wrong?

[cutie.]

just spam the button
the hitbox is generous.

[SilverSet]

Aim under a specific angle, the higher you aim, the higher you will fly. Try it out on some big and open maps, like Paradice. After that, you will get used to it.