Line-in signal works only with effect (KP3+)

[GON]

Hi, i have such problem with my KP3+
Sometimes when i connect microphone to yamaha mixer and use it as line-in signal on KP3+, i can hear the original sound without touching the pad. And sometimes when i use smaller mixer just for mic preamp, the line-in signal works only while touching the pad.
Can somebody explain to me, how can i make that the original line-in signal could work without adding any effect while touching the pad?

[neotechtonics]

check you haven't switched it to send mode on the back by mistake.