I've probably got about 20% of all the feedbacks I could have. After a certain point, nobody really cares anymore, they get the idea. In the beginning, it's tough because not everybody might leave feedback but if you just keep at it (or just ask everyone who you deal with), you'll eventually get a respectable number of positives. Might not get a star next to your name or whatever, but enough for people to know that you're not inexperienced or constantly flaking.