Are the birds ignoring your finch feeder?
The high oil content that makes niger seed (also called 'thistle') so nutritious for birds also shortens its shelf life. When the oils dry out the seed loses it's food value and palatability. If you bought your seed last winter and now the birds won't eat it you might as well throw it out. Or did you buy it recently at a big box store only to see your finches turn up their beaks at it? Keep in mind that your seed may have sat on a store shelf or in a warehouse for quite some time before you purchased it.
The best way to insure that the birds will eat your seed is to buy small quantities at a time from a bulk retailer - just enough for a month or two. At Jayhawk Audubon we buy most of the seed sold at our sales from Tarwaters in Topeka. The birds are feasting voraciously on the niger I bought at our early November sale! Lawrence Feed and Farm Supply on 6th St in Lawrence also sells high-quality seed in bulk.
One more note...niger seed is not a thistle at all - the seed comes from the Afican plant Guizotia abyssinica, a yellow-flowed daisy-like plant. The seeds are heat-sterilized prior to importation so they will not germinate in your yard.
Four Goldfinches and two Pine Siskins!