My sheep jumping over (or going through) 2.5 high fence wall

I have a sheep pen near a cliff. There is a one set of fences to hold them in. However sometimes I find them at the other side of the fences. And this happens only at one part of the fence wall. Here is the problem. http://imgur.com/lVxViRo. They jump over only at that side of the pen. And this is another picture of the pen. http://imgur.com/4BPq90E Sheep always stay in at that part. So I thouht they may be stepping on each other and jumping over 1.5 blocks high fences. I decided to heighten it and made the fence wall 2.5 blocks high. Did it solve my problem? No. http://imgur.com/NheWWjq . Another unusual thing is I can't lure them in when I break fences. They might be staying to prevent falling but I really don't know how this happens.