ACR X NON ADHERENCE TO ATC CLRNC. FAILED TO DSND AS INSTRUCTED. PLTDEV.

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ACR X WAS SEBOUND FROM 12 NM N OF THE ARPT. ACR Y WAS INBOUND FROM 16 S OF PHL; AND WAS DSNDED TO 20 FROM 60 AND ASSIGNED A 020 HDG. ACR X WAS DSNDED FROM 60 TO 20 AND ASSIGNED A 150 HDG. APPROX 1 MIN LATER I NOTICED THAT ACR X HAD NOT LEFT 60 AND ASKED THE FLC IF THEY COULD EXPEDITE THE DSCNT. THEY REPLIED IN THE AFFIRMATIVE. I THEN CALLED TFC TO ACR X THAT THEY WOULD FOLLOW ON THE APCH. THEY DID NOT UNDERSTAND THE XMISSION AND IT WAS REPEATED WITH THE CREW; THEN ADVISING THE ACFT IN SIGHT. A VISUAL APCH CLRNC WAS ISSUED AND AN ALT RESTRICTION OF 20 OR BELOW 6 E OF THE ARPT WAS ALSO ISSUED. ACR Y WAS ADVISED THEY WOULD FOLLOW INBOUND FROM THE N. AT THIS POINT I NOTICED THAT INSTEAD OF TURNING S ONTO A BASE LEG; ACR X WAS CONTINUING SE; AWAY FROM THE ARPT AND WOULD BE IN CONFLICT WITH ACR Y. I CALLED TFC AND SUBSEQUENTLY TURNED THEM EBOUND. I THEN STOPPED ACR X DSCNT AND ADVISED HIM TO TURN TOWARDS THE ARPT. ACR X RPTED ACR Y IN SIGHT AND WAS INSTRUCTED TO MAINTAIN VISUAL SEPARATION. WHILE SEPARATION WAS MAINTAINED; IT WAS MINIMAL AND COULD HAVE BEEN MUCH WORSE. MY PLANNING WAS NOT WHAT IT COULD HAVE BEEN; HOWEVER I BELIEVE THAT HAD ACR X STARTED THEIR DSCNT WHEN FIRST INSTRUCTED; THE OCCURRENCE WOULD NEVER HAVE HAPPENED. MY BELIEF IS THAT THE FLC WAS NOT PREPARED FOR THIS PHASE OF THEIR FLT; OR IN OTHER WORDS; THEY WERE 'BEHIND THE PWR CURVE.'