Ah yes ... that means LW has detected a corruption in the file & is trying to redownload the parts that do not agree with the hash of the file when you originally chose to download the file. I've had this issue. The reason it happens is because at some point you were shown a message Corruption found, do you wish to continue or stop, and option of always use this answer. Chances are you chose to continue & ticked the option to always use that answer so you might not get the pop up message telling you that LW found the corruption in the file. I believe there is a limit on the no. of times LW will continue to try to download the corrupt parts (I think it's 6.)

Chances are the file is already complete. And the corruption might not be bad enough for it to be used. I did an experiment with a video with same issue, kept jumping back to 46% downloaded. I played back the file & it was fine. So I did an experiment. I made a copy of the incomplete file & moved it to the download folder. Then I continued the download until it did eventually finish. Both files were in good order & no problems with either. So check the file in the incomplete folder (select it from your Library window & press Launch button.)

Else if your problem is different, then it may just be a bad file or a bug with LW. It could be corrupt LW preferences also.