Dispenser for Dirt Conversion

I want to set up a manual clay farm by manually pulling lever of a line of dispensers into shooting water bottles at dirt block, which can then be converted to mud. This would in term transform to clay due to the dripstone block underneath.

However, when I tested this out, only some of the dirt block out of the entire line of dirt become mud. The locations of the dirt block not transformed are also random.

I already checked the inventory of all dispenser having water bottles.