Subdomain takes the position of main site in Google search result

Subdomain takes the position of main site in Google search result - Google Search Console is a free application that allows you to identify, troubleshoot, and resolve any issues that Google may encounter as it crawls and attempts to index your website in search results. If you’re not the most technical person in the world, some of the errors you’re likely to encounter there may leave you scratching your head. We wanted to make it a bit easier, so we put together this handy set of tips about seo, google-search, subdomain, to guide you along the way. Read the discuss below, we share some tips to fix the issue about Subdomain takes the position of main site in Google search result.Problem :

We have one domain and one sub-domain. Until last week both of them appear in first page of Google search for very important keyword. Unfortunately Google dropped our main domain from search results. our main site has been in first page for 5 years!

About one year ago we build this sub-domain. It simply has been redirected to one of pages of main domain.

For solving problem we upload a independent site for sub-domain because we guessed that Google think this is our main page of our site. But problem did not solved.

What should we do? our main site offer main services and we we want that will be on first page.

Shout down sub-domain? Redirect to main site? Put the link of our main site in sub-domain? (About one year ago we put link of this sub-domain to our main site. Google indexed it and continuously bring that to top.)
changing in robots.txt
....

Solution :

The demotion of your home page for a specific keyword can be due to link building using that keyword as anchor text and keyword stuffing. This type of "overoptimization penalty" can cause another page on your site to show up in the search results (but usually lower).

Reference: A close look at what over optimization really is Google SEO News and Discussion forum at WebmasterWorld

If the issue about seo, google-search, subdomain, is resolved, there’s a good chance that your content will get indexed and you’ll start to show up in Google search results. This means a greater chance to drive organic search traffic to your site.