asterisk users - Apr 2007 - can't cancel call conference when invited by asterisk

Hello plp I am a newbie and I have a peculiar problem when asterisk  
invites a user to
join a conference. If a user invited to a conference by asterisk  
cancels the call while
being on conference then asterisk doesn't seem to be getting the BYE  
message and the user
stays in the conference forever until the administrator finishes the  
conference or kicks
the user out. But, if the user joins the conference voluntarily (Not  
invited by asterisk)
then the process works fine, that's to say once a user cancels a call  
while being on
conference then he will diasappear from the conference.

What I don't know is why it works perfect one way but doesn't the  
other way... any ideas
why this is so?.. did anybody ever have this problem  
before?.......thanks in advance