I hunt near a local pig farm in my area. I have experienced the same thing. I've seen a lot of fox tracks in the snow, but no success when it comes tocalling any fox. My personal belief is "Timeing". I have to get there when the fox are there. Try hunting you area at various times, perhaps this may work for you. Obviously the fox are there, thats been proven by the tracks. Now its just a matter of being there when they are.....Just my opinion.
Jake
York PA