I don't see how this is still possible but check this out...
I used to manage a business in the real world and on the back of my office I door kept a sales graph of monthly sales for past years. Every year was the same and there was nothing that I could do about it. It didn't matter if I advertised or not because the sales graph was always the same shape, ie: same highs and same holes. We had new clients coming in from phone directory advertising as well as repeat customers. However I did notice that many regular clients spent more with us at the same time each year so they too could be plotted.
My conclusion was that in the end it comes down to personality. But why is that graph still the same after trading online for more than a decade where no-one actually meets you?
I'm selling from web pages that were written/edited at all different times of the year, so why is March - May still so bad every year that they don't pay the rent?