Changing sources for DSCapture

[tomb18]

I am having a tough time with changing video capture devices in .NET.
I have one simple application that uses dsImageDisplay connected to dsCapture. There is code to get the video devices into a combobox.
I then issues these commands to connect:
dsCapture.DeviceName = "web cam" or "Avermedia capture" or Hauppauge...
then I issue a dsCapture.Start().
This works perfectly fine. I can switch back and forth no problem.
However, when I put the same code in my real program (exactly) I can switch between two of them OK and the other gives an external exception.
In addition, the same program, used on a different computer, behaves the same with different devices. But what is really strange, is that the web cam seems to connect (the light goes on) but the image display is black.
What could I be doing wrong?
Thanks

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One thing that may be important: each of the devices that fail have more than one input. i.e. a vga capture board with VGA in and HDMI in. Perhap the wrong input is being chosen?? How does one selct the right one?
Thanks