I take it you are bottling and letting them carb up at room temperature BEFORE storing them in a cold cellar?
If not, there is your answer. Your yeast has got cold and gone to sleep and its not carbing up and you'll never get a good head.
But if you do, ignore the above.
I also find that young beer has bigger bubbles and the head is always a little disappointing. Think lemonade or cola sized bubbles. But given a month or two, it miraculously gets much better and the bubbles become very very small and being small, the surface tension of the beer holds them together and voila - creamy smooth head. I have no idea if this is particular to me, or if its physics, but i genuinely find the bubbles change over time as the beer conditions.
