White series dimmer and Aux switch

I have just installed an aux switch for a three way switch. The smart switch works fine, but the aux doesn’t work.

I’m using a white series dimmer.

I have:

- swapped the Smart switch with another one to be sure it wasn’t a switch problem

- swapped the aux switch to ensure it isn’t a aux switch problem

- factory reset the switch

- set the switch to dimmer + aux in smartthings and using the padel switch.

I’ve previously had a different smart switch and aux from another company in this location, so I’m confident that the wiring is correct.

I also installed another three way switch 3 feet away and it is working just fine.

I’m very confused