The number one key factor is compatibility of the protein with the other ingredients in the system. For instance, excess free calcium is well known to cause a decrease in milk protein suspension stability and solubility. Sodium phosphate salts are well known to increase milk protein suspension stability and solubility. Probably the second most important factor would be how the powder was dried. With milk protein powders, spray drying conditions can affect how fast water can penetrate the outer layer of the powder particle. If the powder particle is too resistant to water permeation, then the powder is more likely to display poor suspension stability. Third factor would be the temperature of the protein dispersion. Obviously, the higher the temperature when the protein is dispersed into water (within reason…temperatures in excess of 50°C should be avoided), the better the suspension stability.