|Examine the ingredients listed on packages of Nyjer seed.|
When nyjer is fresh, it should look very black. If it's gray or brown, it's old and stale. Goldfinches know the difference and will often reject it completely.
Always inspect bags of seeds before you buy. Look for an expiration date ( not all carry one), seed color and list of ingredients on the package. Vegetable oil is often added to enhance the color of stale seed. The problem is: vegetable oil can become rancid after a period of time.
Birds can be quite persnickety. They like their seed fresh.