Problem controlling gain knob

Hello! Am a beginner DJ and have recently started playing at a club. I use the DDJ ergo with Virtual DJ LE on macbook pro, and I connect it through a line out to an external mixer provided by the club. My problem is that i lose control on the gain knob in the software when I load a new song. I have to turn the gain knob all the way to the left and right and then i get the control back. Any idea why this may be happening? Is it becoz I am connected to an external mixer and the mixer's gain takes over? How do I sort this? And please dont tell me to buy the VDJ Pro..I just cant afford it right now... Thanks in advance for your help!