You might have accidentally overbeaten the frosting so it lost all the volume and became runny 2. Your ingredients were not cold. Room temperature cream cheese would become soft easily losing its stability. 3. Cream cheese is a perishable product so as a general rule of thumb it becomes fluffy with all the mixing, and a lot of air is incorporated in the frosting. But over time it is natural for cream cheese to lose its volume when kept outside especially in warmer climates. 4. And hence is always a better idea to make the frosting when you need it or keep it in the fridge until use and not on your countertop.