Complainers have to be cultivated. Complainers can be your most valuable asset.
And the people complaining should be treated like your best friends: they cared enough to complain. Nobody else cared.
I've found that people don't respond unless they know that you really, really, really care. That's why most of my support emails include lines like "We're here to support you in any way we can so if you run into any problems whatsoever please let us know," or "We'll do everything we can to help you achieve X, so please don't hesitate to contact us if we can help in any way."
I've found that anything less strongly worded than that comes across as formality rather than a sincere offer.
On the other hand I give instant feedback and keep a relationship to any complainers that are my happiest customers now.