What are you using to harvest them?
Like many other resources, what you get depends upon the tool you use. Some will give brimstone, some yield salt, and other, like the pickaxe, will give you nothing. I get brimstone 100% of the time using a pick. I get salt if I accidentally hit a node with a skinning knife or a hatchet.
But if you’re strong enough, I’d recommend just farming steel from New Azagarth. You can get multiple stacks quite easily. It’s faster, less boring and less work than farming materials to make your own steel. Plus you get a lot of other useful stuff.