As mentioned in the below link on how to promote a content type in SharePoint results: How to change the order in which search results are displayed in SharePoint Server 2013, I did the same but the problem is that the number of results are getting decreased when query rule is applied.
For Example: Without promoting query rule, Results are 150, After applying query rule, Results are 10.
Note: The query rule works fine, it's promoting the specific content type to top.