Do you have a salt block WITH minerals? When I was growing up in Minnesota in the 50's our neighbors would buy the white salt blocks for their cattle because they were 25 cents cheaper than the red salt blocks with minerals. We always bought the salt blocks with the minerals and the cattle could utilize their feed better and consequently gave more milk. It may be something as simple as that.
Another trick is to put Azomite in the feed. It only takes a little to be effective. Get it at Peaceful Valley.