Like someone said before, the problem might be in high frequency of keyword. I reduced it to about 2% (checked by seobook.com, yoast show 0,4% or 0,7% kw freq). I really don't know if it's enough(?), because top competitors have this kw on 0,2% - 0,8% level. On other hand, they have also my sub keyword on 5-7% level, and they ranking really well... It's really confusing.