Why do I not always hear ATC read back by pilots (on required instructions)?
The O'Hare Eight Departure is the only departure procedure out of ORD.  This is what it looks like:



Notice that there are three departure control frequencies:


Chicago Dep Con West: 125.4
Chicago Dep Con East/North: 125.0
Chicago Dep Con South: 126.635


Airplanes are usually given the frequency appropriate for the sector that their first departure fix is located in.  If they are very busy, each sector will have its own departure controller and they will be talking to different people.  If it isn't as busy, they pilots will still be talking on their individual frequencies, but one controller may be listening to and transmitting on two or all three frequencies at once.

For instance, if a particular aircraft is cleared over POLO VOR (due west of ORD) they will be given a departure frequency of 125.4 as part of their clearance.  Let's say that the traffic is slow and only one controller is being used to cover all three frequencies.  If you are listening to the East/North frequency on 125.0, you will hear the departure controller issue a clearance to the aircraft, but never hear the response because the aircraft is transmitting on 125.4.