I would suggest you move the tracks you have already downloaded to another folder and delete the original torrent file. I think the client is seeing the partial download and therefore saying "already got this".
Yes it is possible to download individual tracks IF they are individual tracks in the torrent - when you get the box up it should show the individual tracks and you just check the ones you want (they are in this one)
It's not a site problem. It would help if you gave us exactly the messages you get on the screen rather then "it does not download".
As I said, move the files you already have for this torrent to another location, download the torrent file and double click it.
Tell us, step by strep, what happens and the exact wording of any error messages. I have a pretty good idea what the problem is but how to correct it depends on what the client is telling you.
It may also be best to post this in the technical assistance forum rather than here - not many people bar daj read this one you see (except nosey ones like me)