Mobs not spawning in hard, or every difficulty.

So i made an xp farm with a zombie spawner and the spawner only spawns like 5 to 4  zombies, 20 minutes later and there's still 5 zombies. the same thing happens when i go outside at night, i only see a few mobs but in newer areas like a place i've never explored before, a lot of mobs spawn. but after exploring the new spots, mobs never spawns again. please fix this bug because i can't play normally without the xp's.