It is easy to become discouraged when sales are down, or nonexistent. First, remember that different products sell well at different times of the year. We all experience ups and downs.
If you've been designing like mad, and promoting like crazy, but to no avail, here's what I suggest.
Take a good look at your products. Are they customizable? This is very important as Zazzle is a custom site. It's the reason most people shop there. Can you add some templates in the form of text and / or photos? Also ask yourself if they are really good, or do you need to learn more about design and graphics?
If that is not your problem, then ask yourself if your products are unique. Photography is everywhere - and very good photography. Clip art is not in itself unique. What can you do that is different from what everyone else does? It may take a while for you to be able to answer that question. It has taken me years to decide what I want to focus on.
I live near the beach. I collect seashells and write about them on my blog. Now, seashells and beaches are not unique. They are everywhere on Zazzle, and all over the internet. So am I nuts to try to sell this theme? I've had to think about this for a while but my answer is "no". I can do this because my things are unique. I am not simply using beach scenes that everyone has access to through photo sites. I have my own photos to use. I've also created my own elements to mix and mingle to create one of a kind designs. I am taking a popular theme (beaches) and creating my own designs using my own ideas. And they are selling.
I do a little drawing and have been lucky enough to sell some of my art as well. You don't have to be a great artist to sell, but you do need a creative mind. You must find a way to stand out.
If you don't know how to use a graphics program, you should learn. I taught myself and it opened the door to a whole new creative world! Currently I use Pixelmator on my Mac, but there are free programs out there. It pays to learn them.