Edit: Golems cannot spawn on bottom half-slabs either.

Can iron golems spawn on slabs?

The target block and 2 blocks above must not be a full block, nor be redstone-powered, nor be rails, and the two blocks above must not be water. This means the iron golem can spawn inside 1-deep water or inside blocks like slabs, fences, and carpets (if other checks pass).

Why isn't my iron farm spawning golems?

Placing them too high or too low will inevitably ruin the structure and won't allow the iron golems to spawn, meaning that the farm is of no use until you fix the problem. A lack of villagers could also be the reason, as there are at least 10 needed for the farm to work properly and the golems to continue spawning.