I was trying to install mayavi‑4.6.0+vtk81‑cp36‑cp36m‑win_amd64.whl for WinPython64-3.6.5.0Qt5 (Windows 7) using WPCP.
Everything looked normal, the command window (wpcp started via .bat) said OK, as with other packages.

But the list of packages did not show the package. When I used pip install with the wheel I saw that it failed because the compiler was missing.