ZSE CTLR EXPERIENCED OPERROR BTWN FL290 AND FL310 WITH XING AND DSNDING TFC.

Narrative

ACFT #1 WAS INBOUND TO PSC AT FL310. TFC IN THE SECTORS (47 AND 48 COMBINED) WAS MODERATE TO BUSY. THERE WAS TFC FOR ACFT #1 IN DSCNT; ACFT #3 AT FL290; OPPOSITE DIRECTION; AND ACFT #2 EBOUND OUT OF PDX ON J16. THE ACFT WERE STILL QUITE FAR APART AND ACFT #1 WAS ISSUED A PLT'S DISCRETION DSCNT TO FL240. I THINK I GAVE MORE ATTN TO THE OPPOSITE DIRECTION TFC AND MISSED ACFT #2 AS HE CONVERGED WITH ACFT #1. THERE WERE NUMEROUS DATA BLOCKS IN A SMALL AREA WHEN THE CONFLICT ALERT ACTIVATED. I IMMEDIATELY ISSUED FL310 BACK TO ACFT #1 AND ISSUED FL280 TO ACFT #2. ACFT #2 SAID HE WAS OUT OF FL290 AND HAD AN RA. CLOSEST PROX WAS 1700 FT AND 3.2 NM. THERE WAS A DEVELOPMENTAL CERTIFIED ON THE POS WORKING MY D-SIDE WHO ALSO FAILED TO RECOGNIZE THE TFC. SECTOR 47 AND SECTOR 48 ARE SELDOM DECOMBINED DUE TO A POOR DESIGN. I THINK THE ERROR WAS SIMPLY A FAILURE TO RE-EVALUATE ACFT #2 AS TFC ON MY ENSUING SCANS. SUPPLEMENTAL INFO FROM ACN 620131: AFTER EVALUATING THE SIT; MY R-SIDE ISSUED A PLT'S DISCRETION DSCNT. HOWEVER; DUE TO THE NATURE OF A PLT'S DISCRETION DSCNT CLRNC; ACFT #2 WAS STILL TFC VERTLY. I WAS PERFORMING MY FUNCTIONS; COORDINATING AS NECESSARY; AND DISCUSSING TFC SITS WITH MY R-SIDE. WE BOTH MISSED THE MORE IMMEDIATE TFC. SUPPLEMENTAL INFO FROM ACN 620127: THE RADAR CTLR; FOR WHATEVER REASON; DIDN'T RECOGNIZE ACFT #2 AS TFC FOR ACFT #1. 15 CTLRS ASSIGNED TO THE SHIFT. 9 CTLRS ON POS; 1 DEVELOPMENTAL TRAINING. 4 CPC'S ON BREAK. 1 CPC IN TRAINING. 2 OJ'S ON DUTY -- 1 ON POS IN AREA; 1 IN TRAINING.