Natural link building is when other people link to you WITHOUT you doing all the work. How many of your backlinks were done by you, and how many by random people on the internet?
I have the same problem, can i post my site so you can have a look at it? Do you charge for these services and how much?
It could be those junk sites, have been there longer, or are doing tricks Google hasn't penalised yet
maybe you also can do blog commenting on high PR blog...it will give you high quality natural backlinks also......