Short answer: melted (not just soft) butter, no chill, or an over-hot tray.
Flat cookies are close cousins of spread ones. If the butter was melted or very soft when you creamed it, there’s no structure to hold height — you want it soft but still cool. Skipping the fridge rest lets them slump, and placing dough on a warm tray straight from the oven melts the base before it sets. Old raising agent can leave them flat too. Use cool butter, chill the dough, and always start on a cool, lined tray.
Related:bakery-style chocolate chip cookies · glossary:chilling dough,creaming.