Were they American Muffins? If so, they are usually made by mixing dry ingredients in one bowl, and liquid ingredients in another bowl and then combining the two together. If this is the case, you have to do the combiining very quickly, the mixture should look lumpy, and you have to get them into the paper cases and into the oven straight away. It is something to do with the chemical reaction of the liquid and the baking powder, which makes them rise and in turn holds up the fruit/chocolate chips.