Issues with mob spawning in Custom biomes

I am having issues with mobs spawning in Custom biomes at similar rates to vanilla biomes. The mobs will spawn ,but in much lower numbers. I can't seem to find a way to turn up spawning rates. I have one just normal generating biome in the overworld and one in a custom dimension both have the issue of mobs refusing to spawn at similar rates even on vanilla blocks. Any help on increasing the spawn rates for mobs would be great thanks