Ground zone in Proteus/Ares
I realize this isn't a direct answer to the question because it's based on my experience using other software.

If it were PADs or OrCAD, you'd draw the area you wish to be flooded with copper pour (or copper), set the object so that it appeared on the correct layer and finally, you'd set that copper to have a net name. I think this may be what you have forgotten to do. Without a net name it is just node-less copper and won't do what you want. Maybe it's also on the wrong layer? Maybe you have two similar node names and you selected the wrong one?