Transitive dependencies for local aar library
The aar file doesn't contain the nested (or transitive) dependencies and doesn't have a pom file which describes the dependencies used by the library.
It means that, if you are importing a aar file using a flatDir repo you have to specify the dependencies also in your project.
In your case adding transitive=true doesn't resolve your issue for the reason described above.
You should use a maven repository (you have to publish the library in a private or public maven repo), you will not have the same issue.
In this case, gradle downloads the dependencies using the pom file which will contains the dependencies list.