Most likely causes are missing or out of date links, or images that lack the necessary resolution (check the EFFECTIVE resolution values).
Sorry – I should have mentioned that I am an experienced graphic designer with years of experience using Indesign.
The linked files are present and more than adequate in terms of resolution – hence them viewing perfectly when in Indesign.
It is only after the PDF export, that they degrade.
you mentioned PDF/X-1a. That would mean transparency reductions settings are at play perhaps.
And these could influence resolution, if images are affected by transparency.
So I would check:
1. Are there any effects on the images or their containers
2. What are the settings for transparency reduction
Also use some PDF settings where resolution is not downsampled at all.
Turn off downsampling.
And I would test, if exporting to PDF/X-4 would do harm to the resolution of the images.
Just to know, if something basically is wrong with your PDF export functionality perhaps.
I turned off all downsampling in PDF/X-1a setting and it came out perfectly sharp.
Still strange that it started to do this only recently.
Looks like I will have to change all my various presets now.
How did you turn off the downsampling you describe? i am having the same problem....
In the Export Compression tab choose Do Not Downsample from the first dropdown menu under Color and/or Grayscale images. You can also leave downsampling on and increase the threshold in the For Images Above field. If you set it to a high number like 600 then only images with an effective resolution over 600 would be sampled down