Happens to me occasionally, and its very annoying. I checked out one of my sellers profiles, and looked at quite a few of his buyers within the last month. He hadn;t left feedback for any of them. He lost a buyer, because I won t be back.its just a courtesy, and some people can t be bothered.
Many powersellers leave feedback occasionally in bulk. So they may only leave feedback once every couple of weeks. Or, you may have transacted with a seller that really does not care that much about feedback because they have already built theirs up and they won t even take the time to leave feedback for others. Try selling on ebay - then you can build your feedback up!
Some of the bigger (sometimes, newer) people @ E-Bay are either too busy, or simply do not leave feedback due to their large volume of business (or lack thereof). Your feedback to them will still be listed as your feedback left when someone checks yours in the future - so don t sweat it all that much. Good luck - (been on E-Bay since 2005)