Make sure the filenames are all the same, except for the numbering, e.g.
Adventures_Of_Willy_D_1.0.0.part1.rar
Adventures_Of_Willy_D_1.0.0.part2.rar
and so on.
If only one file is named for example
AdventuresOfWilly_D_1.0.0.part4.rar
or something like that, then you will brake the chain.
If that don't help, some of the files is broken, so you have to download it again.