I keep trying to use the spy's ability to undermine or track an opposing army and it won't let me. I click on the spymaster, click on undermine and army, then I click on an enemy unit next to my spymasters and it just doesn't do anything. Is it bug, am I imputing something wrong, or am I misunderstanding some mechanic? I like to use spy to the fullest and other actions work fine just not those two.