Scalloped Potatoes problems, diagnosed

If every slice stays stubbornly firm despite plenty of cooking time, check the sauce for acidic ingredients before simply baking longer. Acid can keep potatoes from softening normally. If only thick slices or the center of the dish are hard, the problem is more likely ordinary undercooking, slice thickness, liquid level or uneven heat.

First look at the liquid. A thin but smooth, milky sauce usually points to excess water or weak thickening. Clear watery liquid with grainy dairy solids means the sauce has curdled or split. Those are different failures and need different fixes.