A few things to double-check that I've seen can lead to an empty Known Plot section.
1 - If your ruler has a low Intrigue, and you've got your Spy Master doing something like technology stealing, then there may be a bunch going on that you simply don't know about.
2 - If you've checked the auto-resolve plots button, then most of them will disappear almost as soon as they hit.
3 - If you're really popular, and you're lacking people who are ambitious or devious under you, then you may just be living the good life.
There may be others - but I've definitely experienced 1+2 keeping things empty for me. 1 ended up poorly (lost my wife and heir to assassination attempts I never knew about), while 2 just keeps things clean if you're busy with other things like war or trying to educate every child in your court.