I'm very much inclined to agree on calling them out on their poor behavior, though I would be more likely to pull each aside privately and phrase it as "is there a reason why you never respond when I wish you a good morning or speak to you in general? Have I done something to offend you?"
This acknowledges that you may have inadvertently done something to ruffle their feathers (even if you are sure that you haven't done so) and shines a light on their poor behavior, but in a private manner.
Of course be prepared for a few sarcastic, heavily acted out "Well Good Morning.. " responses for the first few days. Ignore those and act as though they responded appropriately and they will tire of that game.
