There are a few reasons,
1. The Statement Year 2019, some 2020 or older than that are not supported anymore.
2. Pdf password does not match. (You can edit it on the settings page)
3. Print screens of transactions/online banking/apps are not supported.
4. Real photos of the statements are not supported.
5. Editing the statement with any third-party software. For example, password unlocks.
6. Attached more than 10 pdf in one email.
*If you download the statement from the bank, don't edit it with any third-party software, just send it as the original copy.