Need Kindle Direct Publishing Expert: Gradients not showing up correctly

Hello,

I am trying to fulfill my dream of becoming a children's bilingual book author. I illustrated my pages in AI, exported them as PDFs, and everything looks great when I combine the PDFs into a new file with Adobe Acrobat.

However, when I upload the PDF/X file to Kindle Direct Publishing, my illustrations containing gradients are messed up. Some parts of the illustration are missing color, some have blocks of colors rather than a gradient transition, and some are void of color altogether. I don't know what else to do. It looks great in Overprint Preview, and also looks good when I export it as a PDF. However, when I get it back from Amazon/KDP's PDF preview, color chunks of the image are missing. 

I've done the following:
-Raster at 300ppi
-CMYK color
-Flattened the AI file
-Made sure no images were needing embedding

-Rasterized the whole file before exporting to PDF

I was in the AI community forum, and they sent me over here for Kindle self publishing help. 

Please help. I am so close to finishing my book.