All the components of your stadium area jump from one level to another. And stick to it unless you invest again in them. But when your stadium needs to improve from 1 to 2 and 2 to 3 and so on, the others jump more rapidly (4 to 7 for instance). You just need to improve every component in order to be able to raise your stadium capacity. If you do not improve your youth academy (for instance) you won't be able to improve the rest at some point.