Our birds don't seem to like sunflower seeds?

We have for years been successful with fat balls, niger seeds, and a plastic bottle feeder with mixed seeds (etc) in. Usually 2 litres of the mixed seeds go in a day or so. For various reasons we haven't been feeding for a few weeks (I know we should have been over the cold weather, sorry! so I refilled the fat balls and niger, and I recently bought an RSPB bag of feed which is predominantly sunflower seeds, and mixed this with the mixed seed (corn, wheat, etc) and refilled the plastic bottle. Since then there have been virtually no takers, though the fat balls and niger are as popular as ever. I thought, since I had mixed them, that they might be able to take the bits they wanted, and that some would like the sunflowers. Any ideas, or is there any other reason why