Random webcam failures "red x" icon, inaccessible webcam

Occasionally when doing a video call video on my end or the caller's end
shows the webcam icon with the red x. Other times, it's totally fine - even
within minutes of Windows Live sessions.

The issue seems totally random. Aside from webcam permissions issues, what
other reasons are there for the webcam not to show up intermittently? I
didn't find explanations in the help documentation about causes for this. Is
this a bug or because of bandwidth?

Thanks in advance.