I'm not sure about your saddle, but my Selle Italia Flite had ovalized rails - effectively changing the width and making it so it wouldn't fit some seatpost clamps. I had this issue with a USE seatpost, so there were only three solutions: a new clamp (if you can get one for your seatpost), a new seatpost - I ended up with a Thomson and got rid of that terrible USE, or a different saddle all together.