ML10x Effects Loop

I am using an effects loop with the ML10x as shown on the site, but for some reason i can’t get any audio through the effects loop. I’ve tried multiple pedals in the loop and none of them make a sound.

In my test setup I have a strymon sunset, a microcosm, and tuner in the four cable method.

My setup has:
The Sunset in send and return a (tip)
The Microcosm in send and return b (tip)

If I put the sunset in the tip in/out row it works great.
If i put the microcosm in the ring in/out there is not change.
If i put the microcosm in the tip in/out before or after the sunset it mutes the channel.

I’m not sure what is going on and any help would be appreciated.