Check the container first
Look at the first bytes. A ZIP starts with PK, RAR starts with Rar!, 7z starts with 7z. If the header is HTML, you saved a login or error page under the archive name. On macOS or Linux, `file` and `unzip -l` answer this in one command; on Windows, rename a copy to .zip and try to open it.
A zero-byte file, or one far smaller than the original download, means the transfer never finished.
The archive is truncated or corrupted
A ZIP keeps its index at the end. If the last bytes are missing, the reader cannot list the pages and reports an invalid archive even though the first images are intact. Download again, ideally without a manager that splits the file, and compare the size with the source.
If the file opens but one page fails, that entry is damaged; `unzip -t` names it.
It is actually RAR or 7z
Renaming does not convert. Extract the archive with a tool that understands RAR or 7z, then pack the images into a ZIP and rename it .cbz. This reader does not support CBR or CB7.
The archive is encrypted
A ZIP can protect entries with a password. A browser tab has no place to type it, so this reader stops with a password-protected message instead of showing blank pages. Remove the password in 7-Zip or a similar tool, then reopen the archive.
The images are in a format a browser cannot show
This reader accepts JPG, PNG, WebP, GIF, AVIF, and BMP. TIFF, JPEG 2000, HEIC, and JPEG XL are common in scans and are not decoded by the page. Convert those pages to JPEG or PNG, then repack.
Some CMYK JPEGs display with different colours in a browser because colour conversion is left to the browser. Re-exporting the pages as sRGB avoids the surprise.
The pages are there but out of order, or some are missing
Alphabetical sorting puts 10 before 2. If the reader has no natural-sort option, rename with zero-padded numbers. Duplicate names across chapter folders also confuse the order, and an archive that mixes images with thumbnails can show what looks like a missing page.
It works on a computer but not on a phone
- iOS: the file may still be an iCloud placeholder, so it is greyed out or fails to open. Download it in Files first.
- Android: the picker may filter .cbz by MIME type. Pick it from Downloads, or rename a copy to .zip.
- Both: very large pages can exhaust a phone tab. Pages are downsampled automatically, but an archive with hundreds of huge pages can still be slow to open.
Narrowing it down
Try the same file in another reader. If it fails there too, the archive is the problem. If it opens elsewhere but not here, check the image formats and encryption, which are the two limits this reader enforces.