Premiere elements 12-Difference matte effect doesn't work

I want to extract moving subject from background in Pr El 12. I have 2 clips, a person walking along the street on first clip and the same street without person- I'll call it background. Both clips shot from tripod from the same position and everything is the same except for moving person. I want to extract moving person so that all other pixels are transparent so that I can put it in front of some other background. In theory that is exactly what differernce matte should do. So I put background clip (empty street) on layer 1, street with moving person above it (layer 2) and than apply effect "Difference matte" on layer 2. The result should be moving person without background (person on black bacground since all pixels which are the same on layer 1 and two should be "erased". However that is NOT what I get, I see only the background clip and when I select that clip and turn opacity to 0 I see only black screen. Does that mean that above clip is completely transparent instead of just non moving parts? There are 3 sliders but no matter how you move it result is the same. Am I missing something?