I have 2 pedal switchers and a commander in my rig as well as pedals by various makers. I own a Red Witch phaser which is an incredible pedal. However, when it is in a pedal switcher loop I lose about half the volume when engaged but it passes the signal at full volume when the loop is on but the pedal is off. I tried an ibanez chorus and the voodoo lab chorus in the same loop and both worked perfectly. In my mind that eliminates the cables and the pedal switcher as culprits. I then tried the Red Witch with a guitar straight to amp and it seems normal. I also tried the buffered vs normal inputs on the second pedal switcher (phaser is loop 4 on 2nd unit) and got the same signal loss. Could there be an impedance mismatch or something that is causing the signal drop?

No good reason for this that I can see. Try running directly to the Input of the Pedal Switcher with only the Phaser in a loop, then Out to the amp. Try each loop and particularly try swapping out the cables running to and from the Phaser. How are you powering the Pedal Switcher and the Phaser?