Lemon Curd problems, diagnosed

First look at the curd, then consider its temperature. If it is thin but perfectly smooth and still warm, it may simply be undercooked or not yet chilled. If it is thin and also grainy, speckled or slightly scrambled-looking, more heat is the wrong move: the egg proteins have already overcoagulated. If the curd thickened but later shows a greasy or buttery layer, that is an emulsion problem rather than a setting problem. A thermometer is more useful than a fixed cooking time: King Arthur's current stovetop lemon-curd method thickens the mixture to 170°F, removes it from the heat immediately, then chills it to finish setting.