Since 3.73 we have added a setting to the uhcconfiguration.yml to modify the spawn rate of all mobs and animals. By default, this value is set to 3x. To disable set to 0.
#This by default will spawn 3 more where 1 normally would spawn set to 0 to disable.MOBRATEMULTIPLIER: 3
spawn-limits:monsters: 70animals: 15water-animals: 5ambient: 15
The bukkit.yml includes max spawn rates for animals, try upping the "animals" limit. I recommend 35.
Many external plugins like TerrainControl, Essentials, etc, all modify spawn rates. Make sure you have configured these correctly so mob spawning is allowed. Note: BGHDDevelopment can not configure external plugins for you, please do not ask.
You may also use plugins to increase mob rates like:
Many of FateUHC buyers use custom Spigot forks like aSpigot, wSpigot, and others. These spigots can include custom spigot.yml(paper.yml, spigotname.yml, etc) files which include mob spawning settings. Make sure that you configure these to your liking to allow for mobs to spawn at a rate that you want.