Sadly yes I've noticed the same thing over the winter.
Most of my portfolio is related to sports with the odd travel related bit added and these are generally warmer weather activities so advertisers are always potentially going to pay less outside of peak periods.
I certainly expect my cycling related hub earnings to raise during May June and July on the run up to the Tour De France. I've started concentrating on events and sports that fall at other points in the year which make be more advert specific.
Is your portfolio affected by the seasons or do you think there are key points during the year that will be favourable for advertisers? This hubbing malarky is hard enough to get the traffic let alone make good money from it so you're at least part way there. Good luck. CF