When you go to compile while the editor is open, it first compiles the code and then attempts a hot reload if the editor is open. If the compile succeeds, due to other factors, the hot reload can still fail. Once the hot reload fails once, currently at least, it fails to keep the link between the compiler and the editor. This results in the compile button disappearing and the .dll no longer being updated.
The fact that your code changes aren’t being reflected in the editor doesn’t necessarily mean that the compile itself failed. Even if it succeeds, if the code isn’t linked with the editor and doesn’t update the .dll, you’ll never see your changes. I would suggest trying to close the editor, delete you Binaries and Intermediates folder, generate project files again by right-clicking the .uproject file, and then compiling from inside of Visual Studio. If the compile succeeds, you should be able to see your changes when you go to open the project.
As far as having trouble importing this static library, I would suggest posting on the C++ section of our forums as this topic is not related to that subject and you are likely to get more detailed help for that subject there.
Edit: I forgot to add here, but through the help of another user, I was able to reproduce this issue before and have entered a bug for the problem. You can find the information here: UE-31575