One Page is Not Ranking Correctly

Hello,

I have been customizing my post article SEO (https://missionquest.org/blog/) to target specific words on Google. However, one of my pages is not ranking well, even that the word difficulty is only "6" and the MOZ "grading" of the page is 96%. This is the page: 

https://missionquest.org/how-to-choose-a-missionary-service-agency/

Any idea?

https://missionquest.org/