WIA not returning all the devices

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C#

i am trying to develop a remote image scanning service in c#. when i call the following function, it returns only 2 WIA devices (scanners). But my system (xp) is having 4 WIA devices installed, including 2 wireless scanners (one Brother and one Samsung scx 3200w wireless scanners). when i try to acquire image using other app like photoshop, it shows all 4 devices. pl help- Thanks

public static List<string> GetDevices()
{
    List<string> devices = new List<string>();
    WIA.DeviceManager manager = new WIA.DeviceManager() ;

    foreach (WIA.DeviceInfo info in manager.DeviceInfos)
    {
        devices.Add(info.DeviceID);
    }

    return devices;
}