XCode 8 .hpp file not found

I am starting to learn C++ and XCode. This must be an incredibly easy problem to solve, but is incredibly frustrating. I have created a new XCode command line project. The project is created with a "main.cpp" file. I then proceed to add previously written code modules using File->Add Files to... All of the files are copied into the local folder of the project and appear in the project's tree structure (as shown in the Project Navigator). Yet, when I build the project, the first error I receive is XXXX.hpp file not found. This is not a very large file, it has something like 15 lines in total. Something so incredibly simple must be preventing XCode from "seeing" the header file which I just added. I am at a total loss as to what that is....which makes it difficult to spend time learning C++ and using XCode as the IDE. Just as an additional piece of info, I had created a different project and created new files (using File->New->File) and created the 4 files (2 .hpp and 2 .cpp files) which are contained within the set of files I had added as described above) and they worked just fine. So creating the files within a project session works, but adding them from elsewhere on my drive does not seem to work. I would greatly appreciate any suggestions/guidance on how to resolve this issue.