ORIGINAL: pe reivers
From your first post it seems the cap is not pushed all the way down. Use a twisting pushing motion, and push down HARD so the cap moves over and beyond the plug hex. Then is when the two cap halves close up again.
I know what you're saying, I'm sure it is going past the hex, it bottoms out on the cylinder, but theres still a gap. I can close the gap with the hose clamp, but the interference is still there. Thanks