Like too much sugar in a glass of iced tea, any more thn can dissolve won't dissolve. So... you have to use (or dump) the water with the dissolved salt in it and then add more water to be able to dissolve more salt. It takes up to 4 hours for water to dissolve all the salt it can take up into solution.
Using pelletized salt causes this problem if you don't have a salt grid in the brine tank but it can happen with use of a grid also. Solar crystal salt doesn't do this; it dissolves totally.