This is a very specific bug...
I've made a column of dispensers and I put a bucket full of water in each of them.
When I activate all of them, the water is dispensed correctly. But when I activate them a second time to remove the water, the 6th dispenser (and only this one) don't remove the water. This bug only happends when the block below sources is a block of soul sand or a magma block.
On the screen, there is 3 columns :
right with a regular block (sandstone here)
center with soul sand
left with magma block
Sorry for my bad english, i hope that's understandable with screens.
Could you include a screenshot of the backside of that contraption, so that we can replicate the redstone wiring? Or, better yet, a world download?