...that or you dont have permission to delete them. does it give you a message when you try to delete them? if it says anything about not having rights to delete,rename,move,open the file then you have to aquire rights to the file. pretty easy but it took me forever to figure out how lol.
If its something like "being too full" you can easily check that by looking at your freespace but odds are if thats the case windows will say something like "There isnt enough room in the recycle bin, do you want to permanently delete this file?"