missing DLL, can't find path?

I can't find this covered in the forums exactly.

I'm trying to attach a DLL to my project in a subfolder.  The whole project can be compiled/made without errors.  It's only when I attempt to build into Debug/Release that I get the error:

"the program can't start because XYZ.dll is missing"

If I simply relocate the DLL to the project root folder, then it runs.

I don't get it.  I've got the DLL attached to the project in the tree correctly.