mine went crusty because I put dead sea salts in it...they have a high moisture content and made everything else sticky. I ended up taking out clumps of dead sea salts and then drying the rest on a cookie sheet in the oven on the lowest temp for a few hrs (turn the oven off and on to prevent scorching)
I have never had this happen and I use straight dead sea salt. Mine never clumps really. But do you put added oil or just fragrance.
I like to toss mine in Jojoba for moisture but if I slip and use too much it collect moisture. That's the only thing I can think of or maybe too much fragrance oil?
There is a whole thread floating around here about moisture in bath salt. I saw someone bumped it back to the top just a day or 2 ago. See if you can't find it. Several people contributed to the discussion.