Why does powdered coffee creamer dissolve easily in fresh, hot coffee but not reheated, hot coffee?

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I’m new to using powdered coffee creamer, but I’ve noticed that, with a spoon to stir, it mixes instantly with a fresh pot of coffee but 30 minutes or an hour when I go to pour the second cup, it clumps up and can’t be mixed. Same result when I keep the pot heated and/or when I reheat the mug in a microwave.

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