If it's none of the above it might be the same thing as on mine.
Since we live in the same country it might be that. Anyway, I had parents but no grandparents even on generation 5 but I let one of my tamas grow old, until it became an oldtimer. It mated with another oldtimer and had boys. I let him leave when he was 36 and raised his child and the Parents/Grandparents section was in order. I'm not sure if this could solve your problem but it might.