mob farming-what you should know

[Zedwick]

They're just too close, it looks at the number of animals in near-by and won't allow you to over-breed the area. If you had 2 pens with 30 blocks between them, and in one pen you had 10 cows, and the other you had 3 then you could breed cows in the second pen, but not the first. If later the first pen only had 8 cows in it, then you could breed those cows again.

The error message is supposed to be a little more helpful than that, I'll take a look at it.