Goldfinches

Just a general question about Goldfinches. When I started feeding the birds a while back, I had loads of Goldfinches who loved the nyger seed I put out for them. They used to queue up on the telephone lines overhead to come to the feeders. I am now using different nyger seed than normal and they don't seem to have the same interest in it now? Do birds get used to a particular kind of food, has anyone else noticed it?