Minecraft – Water Elevator does not work in death-pit EXP farm

I found a dungeon with a skeleton spawner, and I'm trying to build a "Death-pit Experience Farm" around it.

With the help of wiki pages and youtube videos, I managed to use water to flush the mobs spawned to a tunnel. At the end of the tunnel, I'm planning to build a water elevator abusing the mob AI, hoping them to swim up 23 blocks and drop down from another pit:

W
L
W
L
W
L                     [ room  ]
W                     [ with  ]
L<---<---<---<---<--- [spawner]   (L=ladder, W=water, <-- stream)

Everything works fine except the mobs will not swim upwards. They get stuck at the bottom of the water elevator. (If I myself follow the stream and hold "space", I'll reach the bottom of the water elevator and swim up smoothly.) Why aren't skeleton swim up? What have I done wrong?