I suppose saying 'it just does sometimes' wouldn't be conducive. But the truth is, I'm not entirely sure, and it does just seem to happen sometimes. It could be the size of your eggs is different, the type of starch you've used (sweet vs. sour), or that the milk mixture wasn't hot enough. You could try adding an extra 1-2 Tbsp starch to the dough, but if it's still very runny, then transfer it to the refrigerator to chill for at least 30 minutes (1-2 hours, if needed), and it should thicken up enough to scoop. However, if you're using a mini muffin tin as I do, then I'd also point out that it doesn't matter too much if the batter is still a bit liquidy – they should still puff up amazingly in the tin.