Generally, if you keep finding the same articles, or see the same authors cited repeatedly, it's a sign that you've found the key articles, but you may also need to look in new places.
Have you checked Subject Specific Databases in your Discipline?
More focused results than a general database
Searched Google Scholar?
A broad search across disciplines and types of sources - discover new resources
Explored Citation Searching?
Find articles that cite previously published work - can lead you to seminal articles - Google Scholar searches current education citations well
Dissertations contain literature reviews and references which can help you find relevent research
Searched Online Educational Resources?
Open access journals, data and statistics, testing, certification, educational organizations
Found Proceedings of Conferences or other Meetings?
In education, starting places to look are: in the government version of ERIC, on the organization’s website, or in Search@UW.
Combined catalog of many different world libraries finds what's available beyond UWM
Looked at Review Articles/Reviews of the Topic?
Use term “literature reviews” or synonym as a search term - some databases allow searches by document type such as "review article"
Consulted Faculty and Colleagues?
Faculty and colleagues are a valuable source of information and direction!
Try these recommended online sources