You probably need a higher Lust cap. There are three levels of max Lust: 99, 199, and 999. When you reach the cap the first two times, you can trigger a scene at home that lets you raise the cap, and doing so unlocks more lewd scenes.What do I need to do in order to have sex with the PE Teacher? What stats do I need?
If you have done that and you are still stuck, you may need to progress the story a bit further.