Aircraft on DFW RNAV departure strays off course conflicting with a departure from the other side of the airport.

Narrative

Aircraft X a H25B departed the west side of DFW (Runway 18L) & veered to the east side & conflicted with Air Carrier Y that was taking off of the east side (Runway 17R). I was training on DR2/3 (the west side) & made a transmission in the blind to Aircraft X as soon as I noticed him drifting to the east. No answer. I then instructed my trainee to try him & if no answer (which is what happened) to call Tower & have them turn him to the southwest (195 heading) & put him on our frequency. DR1 (the east side) stopped Air Carrier Ys climb. We tried again to see if Aircraft X was on the frequency & he was not. I then had my trainee call the Tower & tell them to turn Aircraft X further to the southwest (220 & to switch him to us). Aircraft X finally checked in on the frequency when he was 7+ miles south of DFW. When we were introduced to RNAV departures we asked at our briefing about just such an incident happening when an aircraft departs off of one side of the airport & crosses over the centerline & loses separation with an aircraft on the other side. We were assured that 'this would never happen' yet it happens at least once every 2 months possibly 10 or more times a year. In fact it happened again 2 hours later. They will be processed as pilot deviations and DFW will go on like nothing happened.