It sounds like moisture is getting to your bath salts. How do you store them? I don't add baking soda to my bath salts. Unless you are adding citric acid along with the soda, your recipe won't produce fizzing bath salts.
I include BS and CA in my salts to give them a bit of a fizz. I store and sell them in those stand-up mylar & plastic pouches with both a zip-lock and a heat seal. Previously I had the issue that after a while, unsold ones started to "clump" a bit and look slightly damp. So I threw a silica pack into each one of my last batch, and within a few days they had all turned solid. le sigh