ZDC CTLR ISSUES CLRNC TO SEPARATE A BE20 AND A B733; BUT MISSES READBACKS AND HAS INCONSISTENT COMPLIANCE BY B733 FLC.

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A SLOWER ACFT IN FRONT. I TURNED THE JET OUT TO THE W FOR A DSCNT. ONCE PATH WAS CLR OF THE SMALLER ACFT; I DSNDED HIM; THEN WENT BACK AND TOLD HIM; 'LEAVING FL230; CLRED DIRECT ATR.' I THEN WENT BACK TO PLT (NOT KNOWING THAT I MISSED THE READBACK) AND PUT HIM ON A PARALLEL HDG. THE PLT HAD 1 MIN; 40 SECONDS TO TURN TO THIS HDG. ONCE I NOTICED THE TARGET HDG STRAIGHT FOR THE SLOWER ACFT; I ASKED THE AIRLINER IF HE WAS ON THE ASSIGNED HDG. HE RESPONDED WITH 'WERE TURNING TO IT NOW;' WHICH OF COURSE WAS WAY TOO LATE AND HE WAS INSIDE OF THE 5 MI 'J' RING. I BELIEVE IF I HAD NEVER MISSED THE ORIGINAL MIS-READBACK THAT THIS WOULD HAVE NEVER OCCURRED. I ALSO HAD TO TELL THE PLT 4 TIMES TO FLY A HDG. IF THE PLTS WERE MORE AWARE OF ATC CTLRS TALKING TO THEM; I BELIEVE THAT HE WOULD HAVE BEEN ON A PARALLEL HDG IN PLENTY OF TIME. THIS PLT HAD TO HAVE ALL CLRNCS READ TO HIM AT LEAST 2 TIMES; AND SOMETIMES UP TO 4 TIMES. NO OTHER PLTS ON MY FREQ AT THE TIME HAD ANY TROUBLE HEARING ME. JUST APPEARED THAT HE WAS NOT LISTENING UP.