I agree, the number of sales a day is probably governed by a Poisson Distribution. Without going into the maths, there is a chance of $0 sales for any clickbank affiliate, although the chance of it happening diminishes as you are more successful. However there is always a good chance of making a lot less than average sales on a day. And good chance of a blinding day too. Think of it as how many fish you catch a day - you can improve you rod and bait but don't judge everything by one day's catch.
I remember the day when I got over the mental road block, and stopped caring if clickbank was screwing up, stealing commissions, or whatever else I had been assuming for almost a year. Once I stopped thinking about it and just got to work. good things started happening.