ACFT X FAILED TO MAKE A TURN ON THE DARBS 1 STAR WHILE DSNDING AND STANDARD SEPARATION WAS LOST WITH ACFT Y.

Narrative

ACFT X WAS ISSUED VECTOR BY ANOTHER CTLR TO JOIN DARBS 1 ARR S OF SZW. ACFT AT FL330 AT MY REQUEST. ACFT Y WAS NWBOUND ON RADAR VECTOR DIRECT VUZ VOR. AT THE POINT WHERE IT APPEARED THAT ACFT X WAS ESTABLISHED ON THE DARBS 1 ARR; FURTHER DSCNT CLRNC TO FL290 WAS ISSUED. WHILE ATTENDING OTHER ACFT; I OBSERVED CONFLICT ALERT INVOLVING BOTH ACFT X AND Y AND A VECTOR OF 220 DEGS WAS ISSUED TO ACFT X AND TFC WAS ISSUED. ACFT X WAS TOLD TO EXPEDITE TURN AND DSCNT. PLT DID NEITHER; TURNING NO FASTER THAN STANDARD RATE DURING ENTIRE EVENT; AND LEVELING AT FL310 BEFORE INITIATING A CLB DUE TO TCASII RA AT THE POINT OF CLOSEST PROX TO THE ACFT Y FLT. THIS MANEUVER PUT THE ACFT X IN DIRECT AND IMMEDIATE CONFLICT WITH ACFT Z FLT THAT WAS 20 MI IN TRAIL OF ACFT Y AND ALSO AT FL310. IF THE PLT HAD DONE AS INSTRUCTED REGARDING EXPEDITIOUS TURN AND DSCNT; THIS WOULD HAVE BEEN A SIMPLE NAV ERROR RATHER THAN A DEV RESULTING IN LOSS OF SEPARATION. IN MY OPINION; TCASII SHOULD BE TURNED OFF IN CLASS A AIRSPACE THAT IS RADAR CTLED.