Its waiting for a reaction to complete. Some times a mineral salt may need a day or more to completely disolve. Your dilutions also, need time to become homoegeneous. Many reactions complete faster in warmer solutions or with agitation. When the visible granules disappear, add water to make your full unit quanity and shake or let stand for several minutes or more.
Endothermic reactions require heat.
Some salts will cool the solution when they are disolved. Very Pure Water Water will disolve almost anything, so, completely pure water is a lofty ideal. Rain water has about 400 ppm of impurities. Average well water has impurities in the 2,000 ppm range. A good hydroculture solution should actually be within that range.