Here are the causes I can think of: * The temperature was not high enough with your oven. Try baking it at slightly higher temperatures. * They needed more time to cook through the center because they were large. * The meringue was loose. It happens when: - The temperature of the syrup was not high enough. - The meringue was not whipped enough before adding syrup. - liquid or oil mixed in the bowl, etc. * It took hours before consuming it. Store them with a silica gel to soak up moisture in the air if you don’t eat them immediately.