Empty 2D Histogram Draw

Hi,
I've created a 2D histogram, by adding some others histograms. When I plot it, I can only "see" something using the "lego" option, otherwise I just get some empty plot, I would like to use the "contz" option instead. Why is this happening? Is there anyway to force the output to show up?, I know the histogram is not empty but the lego plot is not the one I would like to have.
thanks,
Jorge

part of the code I'm using:

    TH2F *FPlus = (TH2F*)hist_pp->Clone("result");

    FPlus->Add(hist_pp,-1.0);
    FPlus->Add(hist_pp,0.25);
    FPlus->Add(hist_pm,-0.25);
    FPlus->Add(hist_mp,-0.25);
    FPlus->Add(hist_mm,0.25);
    FPlus->Draw("lego");