Animals have stopped spawning in my world...

Hi, I have recently decided to enclose all my sheep in a enclosure and dye them all different colours... I then realised I did not have enough sheep for all the colours, so I went in search of more animals to push back to my base. I have searched the whole map ( which I know VERY well as it is my main world) and can't find any more animals; they have stopped spawning in my world, not just sheep but ALL animals, also no cows have ever spawned on the world. Hope you can sort it out! Thx