I actually remember hearing about a glitch that stopped people from progressing in one of those levels. Only happens in emulated versions, I believe.
You can try exiting the level, or just that 1 map (if possible) and then saving and reloading.
If that doesn't work, tell us the version of the ROM and whether or not you've patched it yourself.
It's an excellent game, btw. Good taste, heh heh.