It look like it needs a tiny tinsy bit of tightening. There is a washer which needs to form a seal. I mean you could test it, i.e. run your loop for 24hr and check for leaks.
Don't force it! If you have to exert too much force then you are going to break it.
I would undo slightly the other screws, I mean 1-3 turns as to level the gap all around and try again, 1 turn at a time on all screws.... ensuring the gap remains level
Again don't exert too much force. These things can snap very easily.
)Put your barbs on it, put your finger over one and blow into the other. If it is not fully sealed air will come out, no need to set up a loop to test it.