Exactly how are you mixing the salt?
I'm sure you know this already, but just in case. Add the salt to the water, not the water to the salt. What happens is you put the salt in first, you instantly create a saturated solution which you then dilute. Potentially this can create a super-saturated solution in which certain solids precipitate out, which are no longer soluble in water. You also need to mix the solution very quickly. Supposedly there are also solids that if not dissolved right away, run the risk of never dissolving.
Craig Bingman did an article some time back about the "best" way to mix salt. He suggested going to the hardware store, and getting a paint mixer attachment for your drill. Use your drill and this whomping big egg-beater basically, and mix up your salt and then you're done mixing in a matter of minutes.
Try this link:
How to Mix a Batch of Synthetic Seawater in Under Five Minutes