Question from a Noob

I'm 3/4 of the way through season 3. Prior to this I've only played Arusu Install and Rinne Utopia. I have the first 2 light novels but haven't gotten around to reading them yet.

My question:

Why do some spirits like Tohka and Yoshino not seem to have any memories but, Kurumi, the Yamai sisters, Kotori, Miku, and Natsume all have memories. Is there a stated reason eventually?