First, did you receive feedback from reviewers of your two rejected articles? Most journals provide reviewer comments for articles they reject. If you did receive feedback, you should seek to understand all of their objections and suggestions (if any).
Second, if your Graph Theoretic approach is truly unique and adds value over Matrix Theory (and other existing approaches), then it is up to you to make this explicit in your Introduction and Method sections. (As Frederico advised) What are the advantages and limitations of the Matrix Theory approach? How does your Graph Theoretic method overcome these limitations? What new does it add?
Third, you might have to admit that you are wrong -- that your Graph Theoretic approach is not sufficiently different from Matrix Theory or similar. Please do consider this alternative before you proceed. The burden of proof is on you to demonstrate the uniqueness and added value. If you can't articulate it clearly and simply, then maybe it isn't there.
I suggest that you rewrite your paper with these goals in mind. And then, before you submit it another journal, you send it to two or three colleagues who will be skeptical and critical of everything you write. You want people who are willing to pick apart every sentence, every equation, every diagram, every reference. Only after you go through one or two review cycles with these colleagues should you submit to a journal again.