depends on many factors such as - keyword density - PageRank - quality of the website/content - anchor text
I think it depends upon the quality of content and keyword rich in the content and backlinks Thanks A+ couse London Video production marketing
Related Content, PR of the page -- if it allows Do-Follow link is definitely valuable for increasing keyword density and so SERP.
it could be from many things. how "spammy" the links are (non-relevant content, for example) on the page you want backlinks to how much content that page has. Some blogs are just links and have minimal content, for example. You'd be penalized (eventually) in that case. Just examples of "why"