Recently Google changed a small paragraph in the product rich snippet guidelines section that you find here.
The change is the following. From:
When using product markup, the main topic of the page should be about a specific product. Product markup on listing pages is not supported.
to:
When using product markup, the main topic of the page should be a specific product, not a category of products or a list of products. Product markup on listing or category pages is not supported.
