I've made banana bread several times and I can't seem to understand why it rises nicely, and even when I've taken it out the oven the 'rise' stays but then once it starts to cool it falls flat. What is the reason for this? Am I over mixing at the end or is there too much moisture? Any help would be appreciated. Thank you
Usually banana bread rises well as the bananas in the mixture tend to give the batter some extra "lift". There are a variety of reasons why yours may be sinking on cooling but it is most likely that it isn't quite fully baked. Banana bread tends to form a firmish crust on the outside, so even if it looks baked it may not quite be done which means that it will sink as it cools. Make sure the top is firm to the touch and that a cake tester inserted into the centre comes out clean. If the banana bread is browning too quickly you may need to reduce the oven temperature slightly and bake the bread for slightly longer, or cover the top of the bread with a piece of foil for the last few minutes of cooking to reduce the risk of the bread scorching.
Make sure that you are using the correct amount of banana as too much will increase the moisture and make it difficult for the bread to cook through properly. Also measure the leavening agents carefully (baking powder and/or bicarbonate of soda/baking soda) using proper measuring spoons. Too much leavening can cause the bread to rise up too much and this will cause it to sink back as it cools.
If you are having problems still then you could try making banana muffins instead. It is easier to make sure these are baked through properly (again use a cake tester to check).