When there's block on a fence, animals might get stuck under the block, and get out of the cage

In my minecraft world i have a cage for my sheeps. It's in building, and around fence there's carpets and slabs. Sometimes animal might walk towards the fence, and get stuck underneath the fence. Then, it gains ability to just simply walk out of my cage. Also, i thought it may be because small ones go under the block, and when they are fully grown up, they can get out. Here's some screenshots: