I have a problem and i cant get my head around why it wont work the way i do it..;)

i would like to add a bit of sidechain on an fx channel with a reverb. so i set up the sends "kick" (that sends the signal to the compressor) Audio 1/2->3/4. Set the detector input of the reaper compressor to auxiliary input. so far so good the compressor receives the signal. but since this is only a fx channel as soon i send some audio to it, it does not receive the ducking signal anymore. i send the audio like this 1/2->1/2. maybe it sounds a bit cryptic but im sure this is doable, and im just too stupid..;) i would really appreciate some help..