I spend WAY too much time looking at my stats - keeping track of daily views, # of sales, $, etc (I majored in math...what can I say? :)
Obviously, since I sell ornaments, my views/sales are extremely high during the Christmas season. But people do buy ornaments year-round (birthdays, weddings) and I clearly see that my slowest month is Feb. TheSales gradually increase as the year goes on. If you are listing new things often and are, in general, active in your own promotion, then you will eventually get the sales. With all the variables, it's hard to really know what can cause sales to inc/dec... do you now have more competition, have you changed your pricing structure, are the things you make still as popular, are you advertising as you once did, has your views/sale ratio changed (why or why not), and on and on.
As others have mentioned, use your slower times as times to create and develop new ideas/projects!