Did you change your render distance recently? A still-unfixed bug from 1.7.4 (actually, much earlier but it did not affect singleplayer until then) causes mob spawning to be reduced, sometimes to the point where almost nothing spawns, if it is less than 10.
Also, are you in singleplayer or multiplayer? The latter requires that you are either the only player online or everybody else has spawnproofed the areas around them and/or are not filling up the mob cap for a spawner to work well (it is okay for there to be multiple spawners running as long as no more than 70 mobs exist per player, less if they are close enough to load the same chunks). Note that render distance does not affect multiplayer, which is affected by the server's view distance setting and is not something that you can easily control unless you are the owner.