I know what you mean. I always leave the appropriate feedback for any transaction I deal with. I have 58 feedbacks that have been left for me (all positive), and I can guarantee that it only accounts for at most 1/3 of the transactions I've had on ASC. It really pisses me off, because I've always go the extra mile to make any transaction with me, buy or sell, go as smoothly as possible and as quickly as possible. My current feedback reflects that already, but the sheer number of missing feedback is staggering.
In the last year or so, once a transaction is complete (items have been received, etc), I always PM the other party and inform them that I've left them the appropriate feedback and would appreciate they do the same for me. That gets more responses, but still about 1/4 of transactions end with no feedback left for me.