Context menu methods in shell extension aren't being used - c++

Question

I am trying to write a shell extension that adds a menu item to the context menu for all file types. The code for the extension (in its entirety) is located here but I'll include some of the relevant parts below as I describe the problem.
The ContextMenu class implements the IShellExtInit and IContextMenu interfaces:
class ContextMenu : public IShellExtInit, IContextMenu
{
public:
// [...]
// IShellExtInit methods
STDMETHODIMP Initialize(PCIDLIST_ABSOLUTE pidlFolder,
IDataObject *pdtobj, HKEY hkeyProgID);
// IContextMenu methods
STDMETHODIMP GetCommandString(UINT_PTR idCmd, UINT uType, UINT *pReserved,
LPSTR pszName, UINT cchMax);
STDMETHODIMP InvokeCommand(LPCMINVOKECOMMANDINFO pici);
STDMETHODIMP QueryContextMenu(HMENU hmenu, UINT indexMenu, UINT idCmdFirst,
UINT idCmdLast, UINT uFlags);
}
All of the methods are rigged with MessageBox() calls to indicate when they are called. There is only one small problem — they are never called.
The extension includes another class that implements IClassFactory. The implementation for IClassFactory::CreateInstance() is as follows:
STDMETHODIMP ClassFactory::CreateInstance(IUnknown *pUnkOuter, REFIID riid,
LPVOID *ppvObject)
{
if (pUnkOuter) {
return CLASS_E_NOAGGREGATION;
}
if (!ppvObject) {
return E_INVALIDARG;
}
*ppvObject = NULL;
ContextMenu *contextMenu = new ContextMenu;
HRESULT hResult = contextMenu->QueryInterface(riid, ppvObject);
contextMenu->Release();
return hResult;
}
I can confirm that this method is being called and I can also confirm that ContextMenu::QueryInterface() is then being called with riid set to IID_IContextMenu.
However, nothing happens after that. None of the IContextMenu methods are called. Neither is IShellExtInit::Initialize(). Why is this happening?
Test Environment
Visual Studio 2013 Express
Windows 7 Home Premium x64
Edit: I completely forgot to describe how the extension is being registered. The DllRegisterServer() function creates the following registry keys/values and checks the error codes to confirm that they are created:
Key: HKCR\CLSID\{CLSID}, Value: NULL, Data: NitroShellExt
Key: HKCR\CLSID\{CLSID}\InprocServer32, Value: NULL, Data: [path to DLL]
Key: HKCR\CLSID\{CLSID}\InprocServer32, Value: ThreadingModel, Data: Apartment
Key: HKCR\*\ShellEx\ContextMenuHandlers\NitroShellExt, Value: NULL, Data: {CLSID}
Key: HKLM\Software\Microsoft\Windows\CurrentVersion\Shell Extensions\Approved, Value: {CLSID}, Data: NitroShare Context Menu
...where {CLSID} is {52A10783-C811-4C45-9A3D-221A962C8640}. I then use regsvr32 to register the extension and regedit to confirm that the keys/values do indeed exist.