There are several possible reasons for this.
- Make sure you are linking to template products.
- Make sure that all of the template products you want to link to are in the same store category.
- Have you declared your domain so that Zazzle knows to allow images from the image source? If not, read this article - https://help.zazzle.com/hc/en-us/articles/360033430774
- Examine the link's parameters and the values associated with each for typos.
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