I can't have a webcam/ vide/ audio conversation

I don't know why, but I don't seem to be able to have a
video conference or a webcam conversation or even audio
conversation....I can send instant messages, but nothing
more...sometime I get a message saying that my
computer/network settings/Lan provider do not allow
that...well I know that the Lan provider does allow this,
other people seem to do it, yet I cannot figure out what
is wrong in my case...please if you have some suggestions
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