Ensure that your internet connection is running properly. Check your connection status in the toolbar.

Make sure that the file size isn’t too large for the email service you’re using. Some email services have maximum attachment file size limits, which could be causing the problem.

Sometimes, the Firewall on your system could prevent Outlook from downloading attachments. Make sure to update your Firewall so it will allow such activities.

Check to see if any updates exist for Outlook on your computer. Download any available updates to keep the software up to date.