Mob cap filled?

We have a snapshot server running 14w11b and yesterday we noticed a severe lack of hostile mobs. walking around at night i only see 3 or 4 mobs at a time. when they are killed another 3 or 4 spawn. we have a natural mob spawner above y185 and 1 or 2 mobs are spawning very irregularly. sometimes up to a 15 minute wait for them to spawn. we have changed the server to peaceful and back to hard again to possibly clear out any "stuck" mob problems but it hasnt helped. everything was running as normal up until yesterday.