I have a physical peculiarity. (OK, OK, _one_ of the peculiarities I have is physical.)
My lower back doesn't bend forwards. I can arch it backwards, but it won't go beyond board straight when I bend forwards. This means, for example, that I can't touch my forehead to my knees (with legs bent). I am very bendy in all other ways.
I wouldn't care, but it makes rolling hard. I come to a stop resting on my lower back. I can't make a nice circle arc across my back, and we know how important circles are in aikido. If I roll faster I jar myself -- I get round, but it doesn't feel right.
I have searched for exercises that will help me to gradually stretch my lower back _forwards_ in order to gain a curve. I have not found any. All sports and stretching books seem to concentrate on stretching _backwards_.
I want to be able to slouch! (It would make sitting in those hard plastic moulded chairs possible too. At the moment I slide out of them.)