Muffins not as expected!

Hello Lindsey. I’ve just made Delia’s Marmalade muffins, I followed the instructions to the letter but found they rose nicely in the oven for the first 8 minutes or so but gradually as time went on the top centres started to dip. I didn’t open the oven at any stage during cooking and only observed through the glass door. At 25 minutes I removed from the oven and found the tops were slightly overdone but the centre of each muffin very moist as if the cake hadn’t cooked properly ie they’d gone sad. I have an electric fan oven so set the temperature for 180°C. And cooked them for exactly 25 minutes. They were light, and the flavour pleasant tasted but far too moist in the centre. What could I have done wrong.