Even plain leather will sharpen a knife (most steels anyway) it will just take a *LONG* time if the knife is dull, in general when sharpening the more grits you skip the less efficient the process. Thus you generally want a knife fairly sharp before you try to finish it on leather. Some times though the edge can just be slightly rolled and not worn and a light stropping can realign it, so give it a few passes and see if anything happens. If you see no change you may need to rework it on an actual hone and then try the leather again.
-Cliff