Fussy Sparrows

Hi All, I've been feeding a flock of sparrows for a few years with RSPB no mess, but lately the fussy birds have been ignoring a lot of the sunflower seeds, increasing the waste. I am curious as to why they should be doing this, as they are supposed to be the most nutritious part of the mix! I have looked for a more grain based mix, but it seems wheat is just a filler. I am hoping it is just a phase. Thanks in advance for any feedback!