I have made Nigella's Madiera Cake four times and the first two times it was absolutely perfect, however, the last two times it didn't rise or "crack" down the middle at all! I feel rather desperate about it all since I have followed the recipe and the instructions to the letter! Please help!
Nigella's recipe for My Mother-In-Law's Madiera Cake (from Domestic Goddess) is a fairly classic Madiera cake that is made with self-raising fliur and is baked in a loaf tin. If you have made the cake twice successfully and then the last two times not so sucessfully then most likely something has changed in your kitchen over this period.
We would suspect that the self-raising flour that you are using could have gone out of date during the time you have been making the cakes. Self-raising flour contains baking powder and baking powder will become less effective over time. Unfortunately self-raising flour tends to go out of date more quickly than a container of baking powder as the flour is usually stored in a bag which means that the flour will absorb water from the air around it more easily and this affectes the effectiveness of the baking powder. Check the "best before" date on the bag of flour before using it. If you have very humid conditions you may also find that the self-raising flour loses its rising ability even more quickly.
The other most likely explanation is that the oven may be running slightly cool and so the cake isn't rising properly. Use an oven thermometer to check the temperature of the oven and also check the oven seals to make sure there are no gaps. Also don't open the oven door too early as this can cause the cake to sink back.