Which resume file format problems cause the most trouble with ATS uploads?

The problems that most often stop your information getting through are about how the file was made, not the format name. Watch out for:

• Image-only files: scans, photos, screenshots or PDFs "printed as image". No text means nothing to parse.
• Security settings: password protection or copy restrictions can block reading.
• Unsupported types: .pages, .pptx, images, zip files and links.
• Oversized files: images and scans can push you over upload limits.
• Hidden leftovers in Word: tracked changes, comments, hidden template text or auto-updating fields.
• Converted files: PDF-to-Word conversions full of text boxes.

A quick prevention routine is to export directly from your source to PDF (or save as .docx if Word is requested), run the copy-and-paste test, check the file size and use a simple file name. That covers almost every format-related issue you are likely to meet, whichever system the employer uses.