my web cam is built into the laptop, i can send and receive images with skype but not with yahoo, why is it so

originally the cam did work with yahoo, for some reason it stopped sending and receiving with yahoo messenger, yet works fine with skype and msn, this is the case with both my laptops, both acers with built in cams, different models, one 6 months old, the other more than 12 months old, have reinstalled yahoo messenger, installed different versions, beta, 8, 9, vista all to no avail, have changed preferences, checked with m/s explorer to see if it works ... all checks out but will not send, cam opens on screen, says it is connecting ... waiting to reconnect ... connecting ... network error, when i invite someone to view the invitation to view does not show in the conversation pane, can anyone help?