Your file may have failed due to low resolution elements. Diagnose the problem by downloading and checking your Advanced file report. The report will give you a visual indication of the elements that have failed the resolution check.
If the element that is highlighted isn't an actual image and is vector or a drop shadow/transparency, you can fix the issue by re-exporting your file from InDesign. Make sure to set Transparency Flattener to high. This setting can be found in your InDesign PDF export settings, under advanced.
If the highlighted element is an actual image, check the actual resolution in the file report, then upsample the image to the correct resolution (or source a higher resolution replacement) and re-export your file.