Mobs won't pathfind on slabs when there is a block on top of them

Certain mobs wont path find on slabs when there is a block on top of them.

place half slabs down in a line and put blocks so they cant go off the sides then put a roof 2 blocks above the half slabs at the bottom

mobs I've found that cant path find are vindicators creepers zombies and zombie pigmen