If the file is incomplete & you have checked it via previewing the incomplete, then just need to keep downloading it. The problem 'might' be related to both LW & FW using same download folder/incomplete folder if they are. Not sure.

If the previews of the incomplete file shows it from beginning to end & plays fine, then take above advice in my link, you can cancel the download, then remove from the incomplete folder.

BTW which version of LW & FW are you using? (a) If they are different versions then that might cause an issue. (b) One of the versions might have a download % bug.
I see 4.17.3 fixed one hash bug, not sure about other versions.