ARTCC CTLR HAD LESS THAN STANDARD SEPARATION WHILE DESCENDING A WDB THROUGH ALT OF LGT. OPERATIONAL ERROR.

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WDB X WAS SBND ON J91 JOHNN OTK LEESE 5 MCO. WHEN HE CROSSED THE ZTL ZJX BOUNDARY; I CLRED HIM TO CROSS 15 MI SE OF OTK AT AND MAINTAIN FL270 (PLT DISCRETION TO MAKE THE RESTRICTION). LGT Y WAS NBOUND CTY-MCN AT FL310 GOING TO DTW. WDB X HAD STARTED HIS DSCNT OUT OF FL330 TO FL270 ABOUT 20 MI N OF OTK. LGT Y FLT WAS ABOUT 20 MI S OF OTK. C/A DIDN'T GO OFF UNTIL THEY WERE 15 MI APART AND WDB X WAS FL310; DSNDING TO FL270 I TURNED WDB X RIGHT TO HDG 200. BY THE TIME I CLRED HIM AND HE READ IT BACK; IT WAS TOO LATE TO DO ANYTHING WITH THE LGT Y FLT. I OVERLOOKED THE WDB X DSNDING INTO LGT Y UNTIL C/A WENT OFF. THE ONLY THING THAT BOTHERS ME IS; WE ARE GIVEN THIS TOOL TO WORK WITH AND TO ASSIST US (C/A); BUT IT DOESN'T WORK 33 PERCENT OF THE TIME. THESE 2 ACFT WERE BASICALLY HEAD ON. THE ALERT DIDN'T GO OFF UNTIL WDB X WAS AT FL310 AND 15 MI APART. IF THE SYS WAS WORKING PROPERLY; IT SHOULD HAVE WENT OFF THE MOMENT WDB X WAS 300' OUT OF FL330; WHICH WOULD HAVE LEFT THEM ABOUT 35 MI APART. I COULD HAVE EASILY AVOIDED THIS ERROR. I'M NOT BLAMING THE SYS BECAUSE I MISSED THE SITUATION. I'M SAYING IF IT IS THERE FOR US TO USE AS A TOOL AND TO ASSIST US; THEN IT SHOULD WORK AT ALL TIMES. SUPPLEMENTAL INFO FROM ACN 93914. WDB X WAS SBND CLRED FROM FL330 TO DSCNT TO CROSS 15 SE OF OTK AT FL270. LGT Y WAS NBOUND AT FL310. THE CTLR FAILED TO SEE THE TFC AND THEY PASSED APPROX 2 MI APART AT CLOSE TO THE SAME ALT. TFC WAS LIGHT DURING THE PERIOD. I WAS THE SUPVR IN CHARGE AND I WAS QUESTIONED ABOUT STAFFING AT THE SECTOR. I HAD 12 OPERATING POS TO STAFF AND 11 CTLRS; SO ONE MAN SECTORS WERE NECESSARY. HOWEVER; IN THIS CASE; STAFFING WAS APPARENTLY NOT A FACTOR. AN OVERSIGHT BY THE CTLR.