3. BAKING SHEET IS TOO HOT
When creating multiple batches of cookies back to back, make sure each batch of dough is placed on a cool baking sheet. Setting chilled dough on a baking sheet fresh out of the oven can cause the butter to melt before the baking process starts, leading to a flat result.
To prevent this baking error, transfer your first batch of cookies onto a cooling rack and place the cookie sheet in the freezer for a few minutes to bring the temperature down.