Probably checksum protection, sounds like it's common on the Genesis.
Checksum, of course, good thinking. Segaretro says the exact thing that happened to me. Thankfully there's a utility on RHDN for this:https://www.romhacking.net/utilities/342/
Now for me to try my hack again...
I changed the $14 to $02, fixed the checksum, and played up to the boss (sadly my savestate from the unhacked version crashed Bizhawk) and killed him in one hit. So yeah, if you wanted to double his hitpoints for some reason, change $14 to $28 and you're done. With that boss, anyway: you'll just have to play the game and figure out which bytes correspond to which boss/enemy.
Although I have to echo KingMike's sentiments:
Just increasing hit count I don't think increases the challenge in a good way.
If it's just the same shit repeated more times, that's only making it more tedious.
The same problem I have with JRPGs. "Okay, I can't reach the next part of the game, better spend an hour killing the same enemies over and over again!" But hey, changing the boss's health is far easier than doing anything else, and now you know how!
Incidentally, the fact that you need to fix the checksum, and that Bizhawk savestates don't work when you do (and it says "I don't know this game, what system is it?!"), really makes things more tedious for hacking. If there's another Genesis emulator with equally good debugging then I'd love to know about it. I tried Gens KMod and I couldn't see a way of making a breakpoint or even looking at the memory. Oh, if only all emulators could be like FCEUX.