toraiz sp-16 audio through issue

When I connect an external sound source to my Toraiz sp-16 via both audio in  inputs and one pad set to "audio through" i have the following issue:

It seems to be that only one mono channel  is receiving the audio input.  the mixer is showing that the input  has only one input signal (instead of stereo).

In comparison to an internal sample where the output  signal send out a 2 channel sound,  the output is limited to the left channel. If I set pan knob to the right, no sound is available.  

I have tested it with my DJM 900 Nexus 2 and my iPhone.  

Do I need to change something or is it function as designed ?

Thx Tobias