Sounds like you've got the same problem I had
HERE
I'm guessing that whatever you've used to couple the two ends together isn't correctly allowing one part of the signal through. You should have a +ve current through the central core of the co-ax and a -ve current through the shielding. The Magic Eye needs both parts of this to work.
---------- Post added at 12:43 ---------- Previous post was at 12:39 ----------
Forgot to say, as you're getting a decent picture it's probably the -ve part of the signal (carried by the braiding around the central core) that isn't getting through.