When connecting an audio device to your Mac, there are two reasons why Ecamm may not pick up all of the device's audio channels.
Echo Cancellation
Most input devices can take advantage of Ecamm's Echo Cancellation feature to reduce echo and background noise. Note however that when Echo Cancellation is turned on, only channel 1 of an audio device will be used. All other channels will be ignored. If you need channels other than channel 1, turn off Echo Cancellation by unchecking the checkbox in Ecamm's Sound Levels window.
Stereo Mapping
Ecamm's Audio preferences include a checkbox called Map Input Channels 1 and 2 To Left and Right Stereo. If this checkbox is checked, Ecamm will only use channels 1 and 2. All other channels will be ignored. When this checkbox is unchecked, Ecamm will mix all input channels together.