Why can't I leave feedback for some buyers even though they have paid and had their item sent well within the 60 day period?

[millway_corporation]

I get this message  Can't leave feedback for the selected listing(s). when I try to leave feed back for some of our customers who have paid and have had their parcel sent well and truly within the 60 day time period. Can't work it out! I hope someone can shed some light on this for me.