Noob Questions

Hello all. I'm pretty new to Crusader Kings 2, and I think I've got a decent handle on things, but two thing keep confusing me.

First off, often when I start a war over a de jure claim, the war "ends inconclusively" because my casus belli is no longer valid. The thing is, their rulers are still alive. Why is the casus belli invalid?

Secondly, I've had a problem conquering other kingdoms. I've invited claiments to other kingdoms to my court, made them my vassal by giving them a county, and then won the war for their throne. Why aren't they my vassal now?

Thanks!