I had the same issue in SOCAL during my initial training.   I found that if I was pointed at the station they would loose me,  but if I turned as little as 10 degrees away from the station they would pick it up again.  I'm guessing the way the antenna was mounted it couldn't get signal around parts of the plane, but off center it could.  Just an idea.

"Also,  having intermittent transponder problem.  Seems to always have problems with SOCAL in LA basin,  but other radar environments sometimes have a problem sometimes not.  Go figure.  I have reply light flashing but they can't see me - other times no reply light.  Tried cycling etc."

Tom McNerney
N54SG