My back up Mac wont respond to turntables using DJM-S9

I use 2 computers to DJ:

1) Early 2011 MacBook Pro 10.10.5 Yosemite (Main comp)

2) Mid 2012 MacBook Pro 10.14.6 Mojave (Back up comp)

With my main Mac I can DJ using my turntables & S-9 with no issue. On my back up Mac when I try to DJ on turntables with the S-9 I can only DJ in internal mode. If I take it out of internal, the turntables will not respond. I have a new residency in San Francisco USA. The venue has turntables and a S-9, and the exact same applies there too. I dont think I've used my back up Mac with any other mixers so its a very small sample size, but my back up computer works fine on my controller (DDJ-SX3). Please help. I'm not sure what to do.