I definitely fall down on responding to comments left on stories months old, however, and then I feel bad when I remember that I didn't comment.

I think this probably works both ways. It's a rare person who responds to an older story in the first place. I for one, and pretty terrible at leaving feedback on older stories, and I've definitely noticed that when I do I get a much lower response rate to my feedback (somewhat of a vicious circle I guess.)

After all of this discussion, I think I might try to get better at leaving feedback more than I have been lately. I keep reccing stories and realising belatedly that I haven't left feedback to the story itself.