You may need to upgrade. Before version 2.6.3, AtoM used a Flash-based multi-uploader for importing multiple digital objects via the user interface. As you may now, support for Flash was deprecated in December of 2020. Because of this, the old method will no longer work in all major browsers.
In AtoM release 2.6.3, we have replaced the old library with a new HTML5 uploader. You can read the release notes here:
Please keep in mind that if you are importing many digital objects or large files, doing so via the user interface may not be the best method. Most web browsers have a built-in timeout limit of about 1 minute, to prevent long running processes from using all available resources. This means that the web browser can time out during large imports, aborting the upload mid-process.
Instead, consider using the command-line for bulk or large digital object imports. I have previously shared documentation links and tips on how to use these methods in the following user forum thread: