Indesign PDF compression isn't working correctly / is over compressing when exporting to a PDF

Hi,

I was wondering if anyone else has experienced this and possibly might have a fix?

My indesign appears to heavily compressing all of the images in my current document. It's a guide that's destined to be distributed online, so I don't need it to be super high print quality - but the current export is so badly compressed that the details of the diagrams are really difficult to read. 

The document is currently set up as an A4 document - originally created with web intent, but I've changed it to print intent and the end result is still the same. 

I've used both lossless / high quality print export options, and more optimised lower dpi options (150, 72dpi) and the output seems to be the same. I've attached some screenshots of the exported PDF view & and the in document view to show the degree of compression that's happening.

Some fixes I've attempted to solve this so far:

- I originally exported the diagrams first as a Photoshop PDF and secondly as high quality jpegs. I initially thought that indesign might be having issues with the Photoshop PDF format (an issue i've read elsewhere), so I exported them as high quality jpegs, 6000 x 4000 300dpi. 

- When that failed, I thought that maybe the jpegs I exported were too high quality, and indesign was struggling to compress them properly, so I optimised them and made them smaller - 3000 x 1000 72pi. 

- After optimising the images and manually sizing them down, the export came out exactly the same as before. 

- I've deleted the previous PDFs, to make sure I wasn't accidentally previewing previous versions of the same outcome. 

I've also created a seperate indesign document and exported the jpegs from there, which have compressed perfectly fine. This leads me to believe it might be an issue with the file itself. However, if anyone has any ideas on how I could fix it aside from recreating the guide from scratch in a new document, that would be amazing. I can do that as a last resort, but it would same me a lot if time!

Thanks,