Half Slabs and Mob Behavior

I have built a skyblock map and use lower slabs everywhere to prevent spawning. One of the first things i did was build a mob spawner, and the mobs fall down onto the half slabs of my island. The fall is not enough to kill them, but leaves them at a one hit status. 

One thing i noticed is the mobs will not attack me on these half slabs. they appear to have no interest in attacking me, don't move but will circle in one spot and watch me. 

I got some sheep to spawn and built a pen for them on the slabs. I fed them over and over without any success in breeding. they didn't move and acted like the zombies. they wouldn't even come to me with wheat in hand. I added another layer of slabs to their pen, making them full blocks now, and immediately they started acting normally again. i could breed them and they ran around and followed me with wheat. 

I got some chickens and using the information i got from the sheep, i made the pen with full blocks instead of slabs. they acted like chickens and followed me and tried to escape every time the gate was open. i had a 20-30 in their pen when i accidentally left the gate open. the chickens began pouring out, but as soon as they escaped their pen and landed on the half slabs, they stopped moving. wouldn't follow me with seeds. literally just a pile of chickens sitting there.