I'm not sure if hostile mobs produced by CUSTOM mob spawners attained by the command block need light or not, so I tried this both with and without light. Basically, I was trying to create a spawner for skeleton mobs in my boss room, and I did get the mob spawner blocks to get placed. But not matter what I do, whether that's replacing the floor with different stone, changing the light levels, or altering the radius of spawning, all the mob spawner will do is nothing. In fact, the little creature inside the mob block actually vibrates when I'm close, almost like its having spasms. It's eerie almost, like the block is trying to spawn but it just can't. Also, I have another spawner earlier in my dungeon that works, and another one even earlier in my dungeon that doesn't work (although this is a zombie spawner surrounded by half-slabs and I know they can't spawn on them, so this one I still need to look into myself a bit). I think this is a bug, and I'm very interested to know what the problem actually is. And I hope that a solution will be procured as well, of course.