Transparency problems with PDF Export

When exporting a PDF, I am finding that some of my images are exporting at full transparency when I have pulled the image itself down to about 80% transparency.

I have other images imported the exact same way from the exact same Ai file that are pulled down to 60% transparency that look fine once the file is a PDF, so it's not like it's totally incapable of doing it. I am wondering if 80% is just not enough of a difference unless I super update the quality.

Wondering if I should just find out whatever color it 'looks' like when it's transparent and just make my graphs that color. 

Screenshots of indesign vs. the PDF are attached.