Help with Boss RC-20XL phase controller

[Korky Blues]

I've been trying to build a pedal to allow me to change through the loops I have stored on my Boss RC-20XL loop station. It would be perfect for live performances, allowing me to switch between two different phases, allowing for more complex songs than a 4-chord repeat etc to be played. I currently have a single button switch which allows me to change up, but does not allow me to change down through loop phases.

I've heard that using a stereo jack lead, I can connect the red wire and the earth wire to one push-to-make/momentary SPST switch, with the white wire and a bridged earth connection going to the other SPST switch, which will inturn allow me to change up and down through various loop phases.

Unfortunately, in testing this, it has proven not to be the case I am able to switch up through loops, but not down.

Has anyone experienced this situation before, or would anyone know how to remedy it?

Thank you for your time and help