Have had the same problem but only on a few occasions. Whenever this has happened, I "kill the download" and then click on the "Library" tab then the "Incomplete" folder. Find the file that would not move to the library and highlight it. Then click the "Resume" button and it should start to download and as you will see it is not finished yet.
Before you kill the download remember how big the file should be when complete and when you go to the "library,incomplete" folder it will not be that big.
Yes, it sounds like some kind of glitch in the program but this is the first version that I have ever experienced it in (2.8.5 PRO) and whenever I have killed and then resumed it has always finished the download and then moved it to the right folder.