In todays arena, I agree with Tvad and Reubent's approach. Tobias's view should work in an ideal world, but many of us can attest to carrying out flawless transactions, submit genuine praise, and get nothing in return even after several polite inquisitive reminders.
I consider myself to be one of the more honest members on the 'gon - honest/conservative grading, provide exemplary shipment packing, and maintain assuring correspondence throughout - only to be disappointed by blatant laziness IMO.
Only afterwards, after reading through their previous feedback history, do I notice it's a pattern with particular members not mutually returning the favor. I'd have at least another eleven responses since I joined back in '04', and though I still consider it a benefit, even with the current negative feedback policy these days, sometimes feel it's "iffy" until well established.
These days, I'll even politely work it into the initial correspondences as part of the sales agreement, that if all goes well, it's importance is known and freshly embedded into their subconscious.
By all means, try the "squeaky wheel" approach.