Some classes may not show up in the Asset Base Class list in the Asset Manager project settings.
Currently the only example I have of this is the QAPawn code class in QAGame. Blueprints derived from it and have "QAPawn" do show up, but the code class itself does not.
The class can still be added by using the Use Selected Asset arrow.
Some blueprints with "QAPawn" in the name will showup, but the code class itself will not be listed.
Select it in the Content Browser in the C++ Classes folder and click the Use Selected Asset arrow in the Project Settings to set it.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-45471 in the post.