Just spitballing here, please do not take this as a criticism of your work, just advice that may or may not be helpful.
I looked at your store and see that you have a lot of the same designs posted individually on different shirt styles or with different departments (mens vs womens) selected. You may want to look at this recent thread and take from it what you will.
You have a lot of designs that are mostly text but they're all flat .pngs rather than editable text. There are 50 shirts in your Hello, I'm From ... Collection. They are all the same design (with some color changes) but with different place names on them. But they are all flat images where one can't edit the place name even by entering the Design Tool. Doing it like this is creating a lot of work for yourself and really limiting the audience who might be interested. If you kept the name tag graphic but instead made the place name over it template text, you could market the same one product to any audience. If zRank is based in part on ratio of # of sales to # of products, and lets say you sell one shirt from that collection of 50, then that's 1:50. But if instead you had one editable shirt and it sold, that's 1:1, much better ratio for zRanking.
Additionally, and just speculation, it's possible that having many same or similar products with same or similar titles is hurting your SEO both on and off Zazzle.
