I was going to say that maybe the fans are thermostatically controlled, I.e., the outside ambient temperature must be above a certain degree. But, since you said the bottom seat fan blows and it was a warm day, then I guess that's not the issue.
EDIT: I just checked my seats, and both upper and lower ventilating fans are blowing. The outside temperature is 31°.
EDIT: I just checked my seats, and both upper and lower ventilating fans are blowing. The outside temperature is 31°.