FO FLYING WDB UNDERSHOOTS DSCNT XING RESTR DUE TO ACFT FMC NOT FOLLOWING NECESSARY DSCNT RATE.

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IN CRUISE AT FL410 OVER CAP VOR; WE WERE ISSUED A CLRNC TO CROSS 30 NM E OF CAP AT FL330. THE FMS WAS PROPERLY PROGRAMMED FOR THE XING RESTR AND THE DSCNT INITIATED APPROX 6 NM SHORT OF THE TOP OF DSCNT POINT; FIGURED BY THE FMS AND MENTALLY CALCULATED BY MYSELF. THERE SHOULD HAVE BEEN PLENTY OF ROOM TO MAKE THE DSCNT IN A TIMELY MANNER EVEN CONSIDERING THE 50 KT TAILWIND. THE ACFT CORRECTLY RESPONDED TO THE DSCNT COMMAND AND SHOWED 'ON PATH' AT FL390. AT THIS TIME; WITH EVERYTHING SEEMING TO BE RUNNING NORMALLY; I BECAME DISTRACTED BY OTHER COCKPIT DUTIES AND EVENTS. I WAS ATTEMPTING TO LOOK UP THE ATIS FREQ AND GET THE INFO; GET OUT THE APCH PLATES TO CVG; HAD THE FLT ATTENDANT CALL ON THE INTERPHONE AND ENTER THE COCKPIT; ETC. UNNOTICED BY MYSELF OR THE PNF; THE ACFT SHALLOWED THE DSCNT RATE FOR SOME STILL UNKNOWN REASON AND WENT ABOVE DSCNT PATH. AT APPROX FL370; THE CTLR CALLED US TO ASK IF WE'D BE ABLE TO MAKE THE ALT RESTR. A GLANCE AT THE INSTS SHOWED ABOUT 5.0 NM TO THE RESTR POINT AND ABOUT 4000 FT TO LOSE. I IMMEDIATELY DISENGAGED THE AUTOPLT; EXTENDED THE SPD BRAKE AND GREATLY STEEPENED THE DSCNT RATE. IT LOOKED AS THOUGH WE WOULD STILL BE ABLE TO MAKE THE RESTR SO I RESPONDED TO THE CTLR WITH THOSE EXPECTATIONS. IN FACT; WITH THE LAG TIME IN INCREASING THE DSCNT; WE WERE A LITTLE LATE IN GETTING DOWN AND I ESTIMATE WE PASSED OVER THE POINT OF RESTR ABOUT 2000-3000 FT HIGH. THE CTLR DID ISSUE US A 90 DEG R TURN FOR TFC AVOIDANCE AND DID LIKEWISE TO THE OTHER ACFT. DUE TO THE WORKLOAD AT THE MOMENT; I AM NOT SURE OF THE HORIZ/VERT CLRNC WE FINALLY HAD WITH THE OTHER ACFT. IN RETROSPECT; THE LESSONS I LEARNED FROM THIS EXPERIENCE ARE: DON'T TRUST THE FMS TOO MUCH. CONSTANTLY MONITOR ACFT PERFORMANCE AND SIT. NEVER LET OTHER DUTIES AND DISTRACTIONS INTERFERE WITH THE PRIMARY JOB OF FLYING THE ACFT. DOWNLOADING THE AUTOMATION TO A LOWER LEVEL IN ORDER TO PRODUCE THE PROPER PERFORMANCE MAY BE NECESSARY BUT MUST BE DOWN EARLY ENOUGH TO GET THE REQUIRED OUTCOME.