When you move the incomplete files to the new computer, you should move the entire folder. The 2 files within called downloads.dat & .bak are your downld preferences. Without them your incompletes aren't worth much. So close down LW. Then go back to the original incomplete folder & move the said files across. Then restart LW.

If you're refering to the one computer but updated OS, then the only reason I can think of is that your comp has crashed whilst LW was open or you shutdown your comp before LW had fully closed down.