I am attempting to create a zombie farm, and the zombies keep stopping right before the bubble elevator. How can I fix this?
Is that a bubble elevator in there? You need to put a block of ice (or better, packed or blue ice) where that exposed floor is.
IMHO, spawner based farms kinda suck unless you manage to find a place where you can be within 16 block of 2-3 spawners at once.
Dark platform farms high up in the sky offer much higher rates. Plus you get all sorts of mobs and drops. Bones, string, gunpowder, etc.