That's the same one I'm using. Check your co2 hose size, I can't imagine the co2 hose being too big. Cause there's no way it could be too small.
I believed your assembly of the co2 hose is wrong.
You have to twist off that coupler thread. Then slide your co2 hose into it, and then plug in the pin from the other side. The pin is a bit rounded off so once you have your co2 sliding in, it won't come off. No way can it come off. Once you have that assemble, then put then use teflon tape.
What I assume you were doing is that you didn't loosen that coupler thread. You just stuck the co2 directly onto that pin sticking out. As a result, when you open your co2 valve, it then put pushes the co2 hose off. It's a assembly error. Here's photos of the correct way. I jus took apart mines to show you.
Take that coupler apart, slide co2 hose pass through the coupler ring.
Plug it snug and tight all the way to where it prevents you from pushing it too far back.
Assembly the tubing onto the thread, teflon tape that thread, as photo shown.
Slide coupler ring to teflon threaded thread, Then just tighten it with a wrench.
Let me know what the problem was. Was it assembly? So I can let other people know if this problem occurs in the future.
Hope that helps.