This is due to an excessive amount of minerals in your water supply (also known as hard water). The abundance of these minerals (mainly calcium and magnesium) prevent, on a chemical level, the concentrate from remaining in the mixture. It is possible, even with softer water, for there to be an abundance of these minerals over time, due to the effects of evaporation at the sump level. If this is a consistent issue, use the Zebra Hardness Testing Sticks to check the level of your water hardness and compare it to the product data sheet for your coolant. Most coolant manufacturers will specify what hardness range their blend can withstand. If your coolant is outside the range, you can use an additive to decrease the hardness, or even consider RO (reverse osmosis) water. Hard water can also contribute to surface grease formation, which is not readily removed via mechanical skimmers since its surface tension is different than that of straight oil. Coalescers would work better for their removal. Grease formations also contribute to premature filter blockage. Other negative factors due to hard water are rust and gummy residues, either on parts or machine interiors.