IAudioClient::Initialize sometimes take several seconds to return

Question

When I use the Windows WASAPI's function IAudioClient::Initialize to initialize a capture stream on an audio device, it sometimes takes several seconds to return successfully, although normally it only takes several hundred milliseconds. The code I used is listed below:

// Create a capturing stream.
hr = _ptrClientIn->Initialize(AUDCLNT_SHAREMODE_SHARED,
                              AUDCLNT_STREAMFLAGS_EVENTCALLBACK |
                              AUDCLNT_STREAMFLAGS_NOPERSIST,
                              0,
                              0,
                              (WAVEFORMATEX*)&Wfx,
                              NULL);

Not sure why the same function calls can vary some much.