Korg kp3+ interface problem - help !

[Icepro]

Hi guys ,

Recently i bought krog kp3 pad but when I plug it to the interface ( RCA outputs to interface input, and RCA inputs to interface output ) i cant hear most of the effects out of my monitors or my speakers , but when i switch the kp3 to direct instead of send and to line instead of mic i can hear all the stuff from kp3 , any thoughts of this issue? So what im using , kp3 + focusright scarlet interface + yamaha monitors and a headphone , im using RCA cables for the output and the input, thank you guys , i will appreciate if someone could help me to understand.