This is perfectly normal and a harmless issue. One block goes down faster as this is where the brine valve is located in the water softener.
The salt is always drawn from the block that is closest to the brine valve, hence it going down quicker.
It is okay for one side to be completely dissolved away before adding more blocks. As long as there is some salt in one of the chambers, then the water softener will make brine and function correctly.