So i was working on my mob sorter and mob farm and when i made a deep pool of water (about 30 blocks deep) and spawned witches to the bottom of the pool. After a long time only one of the witches got to the surface and the rest of the witches stayed at the bottom and swin up and down.
The mobs cannot swim up against the water current forming at the glass surfaces. Build the wall out of solid, intransparent blocks.