Why are some of my products not exported in the feed ?
So... you have recorded all your settings, generated your feed, saw the progress bar tell you that all products have been written to the feed, and when you open up the XML file, there are no products or very few.
What caused this ? It's simple: the count you get on your progess bar when you generate your feed is simply based on all active products in your shop whose main parent category is part of the categories you have checked in the module configuration, "Categories to export" tab.
However, there are a few settings in the module's configuration that may prevent a product from being exported and included in the feed. They are all grouped in the "Product exclusion rules" tab, so just double-check all the values there.