The amount of one substance that can dissolve in another is called solubility. Sucrose (sugar), the main component of Kool Aid has a solubility of about 2 grams / mL of water (or about 500 grams / 8 oz glass of water) This is A LOT! So, all of the Kool Aid should dissolve, you may need to just stir it more, or try heating it up (solubility increases with temperature of the water.)