There could be a few reasons why it's runny, here are the most common:u003cbr/u003e1. Using a low or non-fat mascarpone can cause it to be runny, there is more water in low - non-fat versions. Go for the full fat version. Read more about that u003ca href=u0022https://www.thefreshcooky.com/homemade-greek-yogurt/u0022 rel=u0022noreferrer noopeneru0022 target=u0022_blanku0022u003ehereu003c/au003e. u003cbr/u003e2. Over beating the mascarpone, it can cause curdling and separation of the cheese curds and water, simply whisk until smooth and creamy.u003cbr/u003e3. Not cooking the custard long enough, it should be thick and pudding like when you take it off the stovetop. u003cbr/u003e4. Mixing the mascarpone into the egg custard mixture before the mixture is completely cooled.u003cbr/u003e