Django - makemigrations - No changes detected
Another possible reason is if you had some models defined in another file (not in a package) and haven't referenced that anywhere else. 
For me, simply adding from .graph_model import * to admin.py (where graph_model.py was the new file) fixed the problem.
I was trying to create migrations within an existing app using the makemigrations command but it outputs "No changes detected".
Usually I create new apps using the startapp command but did not use it for this app when I created it.
After debugging, I found that it is not creating migration because the migrations package/folder is missing from an app. 
Would it be better if it creates the folder if it is not there or am I missing something?