I don't use feedback forms - heavens, I'm afraid of what their responses would be. It would seem logical to do it immediately after a transaction, but to do it months or years passed the closing date - do the clients really care at that point?
I have one client who once a month I was emailing sales data for their neighborhood and they recently opted out. Their closing was six years ago. We had a good relationship and I fought for them on several occasions of negotiations. I think because the market is not so good, they are not that interested in what's going on since they are moving anywhere soon; but, if they have just one more child (up to 3 or 4 already), a bigger home may be in order?!!!
Sorry, I just don't use feedback forms. Perhaps it would be better to visit them a week after closing (if they are local) and sit down and talk to them because you can't defend yourself in a returned form.