salt residue at the bottom of salt tank

I recently noted that the salt in my salt tank had turned hard (I had noticed a lowering of the water quality). I cleaned the Venturi (I hope) and removed all the leftover pellets and ended up with a 4-5 in thick layer of salt. The salty layer is not quite solid but certainly harder than mushy. I did not refill the tank hoping that recharging would use up all the salt but it has been a few weeks and the thickness of the salt layer is the same. Looking into the tank I can see that the brine (if any) is quite clear and then after 4-5 inches the white layer starts. At the same time the water is not very soft but on the other hand is not very hard either. Why is the salt not being used up? I find that very strange. I must add that we only used quality Morton pellets. I will appreciate any input.