Plot the normal Overlap of a Manual Selection

Hi, I'm triying to plot the total or the average (both are valid for this matter) normal overlap of a Manual selection of a group of pellets, but the results are allwais=0. If I plot again but all the particles (not a manual selection), I obtain results, but not if I want to plot only a manual selection.

This manual selection is a big group of particles, that has certainly betweem them a minimum overlap value.

What's the problem?

Thanks!