I noticed while watching TOS that they never really said what kind of warhead was used in a photon torpedo. And interestingly, in "The Immunity Syndrome", Kirk had Scotty prepare an antimatter charge to strap to a drone. Doesn't that sound suspiciously like what a photon torpedo is suppose to be? It's not like it is a power thing because they were able to launch the drone and in "A Taste of Armageddon" when power was used up for shields (and full power phasers not available) Scotty was going to use a dozen or so photon torpedoes. But... maybe because there was almost no power left that even photon torpedoes weren't available to use in "The Immunity Syndrome" because it all had to go to that final push to move out of it?