How do I find a control on a dialog that isn't mine?

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C++

MFC

I'm trying to disable a button on a dialog, but I can't seem to retrieve the button handle no matter what I try (Spy++ can see the control and get its ID).

What I have tried:

I can find the dialog handle, and thus get its CWnd. I test changing the dialog title, just to confirm that it isn't protected somehow. I have the control ID from Spy++.

I tried GetDlgItem, FindWindow, FindWndowEx and EnumChildWindows, but just cannot get the button window handle.

Interestingly, EnumChildWindows doesn't find all the children - so what is Spy++ doing that I'm not?