When processing the WM_CREATE message, your call to CreateWindow(L"13class") is still running. Your cw13 variable has not been assigned yet, so you are assigning an invalid HWND as the parent of your buttons. That is why they do not appear.
Use the hwnd parameter of WndProc_dla13() instead as the parent. That will be the same HWND that gets assigned to the cw13 variable once CreateWindow(L"13class") exits.
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