While you can indeed often hear ATC talking to airplanes whose answers you do not hear, there are other reasons besides wattage that might be the situation.
It is not uncommon for one ATC controller to be working multiple frequencies at once. He might simply be replying to a plane on a different frequency. When traffic is light like late at night, center sectors may be combined. That's another reason a controller might tell you to contact HIM - the same guy - on a different frequency, as in "Contact ME on 1XX.XX" as you traverse his airspace.
Also, as in the tragic KDCA mid air, more often than not one controller talks to traffic in his area of responsibility on both VHF to civilian traffic, and UHF to military traffic. Especially in dense traffic airspace it makes full situational awareness tougher for both pilots. One transmit button press can send him out on both frequencies. When you hear your ATC controller talking to what sounds like a military call sign, vs. an airline type call sign, and you hear no answer that's most likely what's going on.
