UR824: Different outs in headphones and monitors

I know this will be a pivotal knowledge, but I was not able to find an answer anywhere so far. The thing is that connecting headphones to the PHONES 1/2 outputs I can hear everthing that is connected to the analog inputs as well as sounds from the USB-connected PC. However, when listening to the studio monitors (connected to the OUTPUTs) only what goes to the the analog inputs can be heard, no sound from PC … (the dspMixFx shows MASTER activity for both, inputs as well as PC sound …)
Please, can anyone help ?? Thanks!
M.