I am having this exact problem in 14w21b when I lower the render distance. I have a zombie pigman farm that was producing a steady stream of them regardless of render distance, then suddenly something changed and I no longer get a single spawn on a 44x44 platform with the distance set to 8. My system has an i5 overclocked to 4.1 GHz, minecraft is using about 500 MB of ram (I set the max to 6GB) and I am getting 120 FPS so I doubt that the computer not keeping up with the ticks is the problem. If I increase the render distance to 16, my FPS drops to about 40, and the problem goes away.
I am having this exact problem in 14w21b when I lower the render distance. I have a zombie pigman farm that was producing a steady stream of them regardless of render distance, then suddenly something changed and I no longer get a single spawn on a 44x44 platform with the distance set to 8. My system has an i5 overclocked to 4.1 GHz, minecraft is using about 500 MB of ram (I set the max to 6GB) and I am getting 120 FPS so I doubt that the computer not keeping up with the ticks is the problem. If I increase the render distance to 16, my FPS drops to about 40, and the problem goes away.