I have made the American Breakfast Pancakes many times, always sticking to the same ingredients, but this time they turned out to be thin. Why? They still taste great!
Nigella's American Breakfast Pancakes from HOW TO BE A DOMESTIC GODDESS usually rise a little while cooking and have fluffy centres. This is due to the baking powder in the mixture, which reacts with the liquids and forms carbon dioxide bubbles.
It is possible that the baking powder was getting old and has lost some of its efficacy, so it is producing less carbon dioxide. Baking powder usually has a "best before" date on the container, but it can expire more quickly in humid conditions. Also the action of the baking powder is fairly short-lived, so if you have made the batter in advance then the baking powder could have made the carbon dioxide bubbles and then petered out, meaning that the pancakes will be skinnier. It is best to make the batter just before using it. If you need to prepare in advance then measure out and combine the wet and dry ingredients separately (for the liquids do this no more than 1 hour in advance if leaving at room temperature) and just blend the two together at the last minute.