A320 AIRBUS FO FAILED TO MAKE XING ALT DUE TO RETARDING THE DSCNT RATE BY THE AUTOFLT FMS.

Narrative

WE WERE CLRED TO CROSS CLIPR INTXN AT 12000 FT. AUTOPLT WAS ENGAGED AND A MANAGED DSCNT WAS PROGRAMMED INTO THE FMGC SO AS TO CROSS CLIPR AT 12000 FT. I WAS MONITORING THE DSCNT FROM FL200 AND APCHING CLIPR. IT LOOKED LIKE THE AUTOPLT/FMGC WERE JUST BARELY GOING TO MAKE THE 12000 FT RESTR. WHEN I COULD TELL WE WERE GOING TO BE CLOSE; I TRIED TO EXPEDITE THE DSCNT BY SELECTING 'OPEN DSCNT' MODE. SINCE THE AUTOPLT/FMGC WERE STILL CARRYING PWR ABOUT IDLE; I SELECTED OPEN DSCNT TO BRING THE THRUST TO IDLE AND THEREBY INCREASE THE RATE OF DSCNT. AS SOON AS I SELECTED OPEN DSCNT; THE THRUST WENT TO IDLE AS REQUIRED. HOWEVER; THE ACFT IMMEDIATELY BEGAN SLOWING TO A SLOWER AIRSPD (ECONOMY DSCNT AIRSPD). WHILE SLOWING TO ECONOMY DSCNT AIRSPD; MY DSCNT RATE DECREASED DRAMATICALLY. PASSING OVER CLIPR; 'ALT CAPTURE' MODE ENGAGED; BUT WE WERE AT 12600 FT. IN HINDSIGHT; IF I HAD JUST LEFT THE MANAGED DSCNT CONTINUE; THE ACFT WOULD HAVE HIT CLIPR AT 12000 FT OR SLIGHTLY HIGHER (IE; 100 FT). MY ATTEMPT TO CORRECT FOR THIS MINOR DEV RESULTED IN A MUCH HIGHER DEV. IN THE FUTURE; I AM CONFIDENT I CAN IDENT IF THE ACFT/AUTOPLT ARE DEVIATING FROM EXPECTATIONS AND IF NECESSARY I CAN INTERVENE EARLIER TO ENSURE RESTRS ARE MET.