It's possible you have a bad batch of salt.
A couple things I might suggest:
1 - Mix it at least 24 hours prior to using it.
2 - Stir the water vigorously while slowly pouring the salt into the water. The salt should be dissolving before it hits the bottom of the bucket. There should be nothing sitting on the bottom when you're done.
To see if you have a bad batch of salt you'll need to test it for Magnesium, Calcium, Alkalinity, and PH. This is starting to sound like the likely suspect.