| I have oak floors. Nail heads come up, I push them down, and they come
up
| again.
| I won't even ask why they come up

Probably expansion and contraction. Do you perhaps
have temperture extremes in the area? No central
heat, or an uninsulated area below the floor?

I've never seen it occur as a major problem, but
sometimes in old houses, nails need to be tapped
down occasionally.