I have had a similar situation in my garden, a full niger seed feeder all summer!! I am not sure why I am not getting them, neither is next door come to that. I know we have a good number in the area as I hear them every time I am out in the garden (usually in one of the two large ash trees in the area), but I don't think I have ever seen them at the feeder!
I wondered whether it was the location at first, but the other feeders in the same area seem fine, in fact they are really well visited. I then wondered whether it is the design of the feeder (single perch bar as opposed to a ring), but the seed does go down, however slowly, so something is visiting it; just very infrequently.
Not much help, but at least I know it isn't just me
Eric