Not sure if you checked, or if this even applies to your particular issue, but don't forget about the recycle bin. When you "delete" files, many times you are just moving them to the recycle bin. You therefore won't see an increase in free disc space. You need to either empty the recycle bin or do a "real delete" of the file in the first place. To do a "real delete", skipping the recycle bin, hold down the shift key when clicking on the delete menu items.