Here are a few common causes and potential fixes for the issue:

Incomplete Download:

Check Internet Connection: To begin with, ensure that your internet works properly. Weaker internet may pause a download or even break it sometimes.

Restart Download: You could try restarting the download as well. Sometimes, this might be a temporary problem.

Clear Browser Cache and Cookies: Perhaps excessive accumulation of cache and cookies in the browser is causing a problem while downloading. Clearing it might solve the problem.

Malware or Spyware: Your antivirus may detect the download as malware. Look out if your antivirus or a firewall is blocking the download. Un-disable them one after the other to check if the problem will be solved by this, but be careful not to compromise security.

File Corruption:

Check File Size: Compare the file size of what you have downloaded with that of the original one. If they don't match, then the file could be corrupted. Download Again: You can try downloading from another source or using a different browser.

Try Downloading on Another Device: Be able to download on another device to rule out the issue.

Software Problem:

Fresh Software: Ensure your browser or download manager is up to date. Updates usually come with bug fixes.

Try another Browser: The issue could be browser specific. Changing to another browser may solve it

Storage Problem:

There's not enough Storage Space on Your Device. There may not be enough space on your device to download.

Check File Permissions: You should be able to write the file in the directory that you selected.

Download Manager Issues:

Close your download manager and reopen it.

Switch your download manager. If downloading with some third-party download manager, try downloading with another.

Specific to different devices or operating systems

Android: Look for download progress which can run in the background and interfere with your application.

iOS: Ensure the device has enough free memory space and that background app refresh is not disabled.

Windows: Ensure there are updates; also, ensure that the antivirus software has updates

macOS: Try verifying the file permission and checking for disk problems

More Tips

Check out Download History: Most probably you might have got an error report or partial downloads in your download history.

Malware: It can be an infection of malware that leads to such issues. You can scan your computer fully with your antivirus application.

Contact Support: The problem is not resolved, but you are free to contact the website or software developer for further assistance.

Follow the above steps and apply them to your situation: hopefully, you will be able to solve the "download not complete but showing as downloaded" issue.