The problem i have found is that when you dispense with a dispenser water over a block of soul sand, then it will crete like an invisible water bubbles block, and you wont be able to plant nether ward.

-Put a dispenser looking to the block of air right over a soul sand block

-Put a water bucket in the dispenser

-Activate the dispenser 

-When the water is already over the soul sand block, activate again the dispenser (it should remove the water)

-And then there is an invisible water column block

-Try to plant a nether ward