There is a warning on the UbuntuOne Music store FAQs
The Ubuntu One Music Store's partnership with the music labels limits the number of times customers can download a purchased song from the music store to three (3). While this gives people some security in case of catastrophe, additional downloads should not be necessary as purchases are backed-up in the customers's Ubuntu One personal cloud. The initial transfer from the music store to a customer's Ubuntu One personal cloud will count as one (1) download. Any synchronization of purchased songs stored in your Ubuntu One personal cloud to any number of your computers does not count against the music store's download limit. Clicking to download again will transfer songs to your Ubuntu One personal cloud again and will deduct from the downloads remaining. Customers shouldn't need to do this though unless they delete the song from their cloud storage.
I assume that as stated above the transfer to your computers does not count as a download this is either an error on UbuntuOne Music's part or you have tried to download these files to your cloud more than 3 times in order to solve this problem.
I neither of these are the case can I suggest you email them by completing the contact form here:-
They are very quick at responding and do provide excellent support (Voice of experience)