Insert cable on pedals connected to DJ mixer, sound only comes out of one monitor?

I have got both the Boss RC-20XL and DigiTech DigiDelay guitar effects pedals for my turntables. My DJ mixer is the Rane TTM 56. The Rane TTM 56 has a FlexFX section, which is the FX loop section. The inputs are 1/4". The insert cables I am using are Y-cables, RCAs (Red being TIP, Gray being RING), and the other end is a single 1/4". I hooked up one of the insert cables to the Effects Send, using the Red and Gray RCAs plugged into RCA-to-1/4" plugs, and plugged THOSE into the Send inputs on the Rane, and plugged the 1/4" end into the input of the RC-20XL. I did the same with the RETURN, except plugged the 1/4" end of the cable on that one into the Output of the RC-20XL. When I switched the FX switch to on though, I noticed that he sound only comes out on the left monitor! When the FX are on DRY, they come out of both monitors, but when the FX are on WET, it only comes out of the LEFT monitor. I noticed the same thing happened with the DigiTech DigiDelay pedal. I'm pretty sure it's not the pedals, it's most likely my cables, right? Thanks.