Hi Gary, don't do anything yet... It may just be the 128kbps bitrate that SoundCloud converts your file to for streaming.
If you go to your SoundCloud page and click on the pencil icon just below the song, it brings up the editing window. From there you click on "Permissions" and the third item along is "Enable downloads." Make sure that it is ticked.
I should ask, however, what quality/bitrate do you upload at? I always upload at 320kbps and make my tune available for download. If you are selling your tunes though, you may want to leave the download option turned off.
cheers
andy