animals escaping

I have created a farm area for my animals, however some seem to be able to escape. I have had two sheep and two chickens escape from their pens made up of fence posts. I am sure i have not left the gates open and there are no blocks inside to the same level as the fence to allow the animals to escape. I have not seen how it happens, just that i get to the farm and notice an animal is loose. all of the times it has been raining.