Where do the seeds go?

I'm trying to collect some teasel seeds for MrsW but I'm struggling. Whatever stage of the teasel heads I look at there seems to be an almost total lack of seeds. I brought back an assortment of heads from those that still had some flower on to the really brown ones, including one from last years stock. We managed to collect 8 seeds from all that lot. We aren't short on teasels around here so the birds can't eat them all the moment they appear. Can anyone explain where they go? 
One for Wildflower Week. Thank you for hosting Miranda, hope I've tagged it right.