AN ACR MLG FAILED TO MAKE ITS XING RESTR.

Narrative

WE WERE FLYING INTO EWR ON THE WARRD 4 ARR AT FL370. CTR CLRED US TO CROSS RIDGY INTXN AT FL240. WE SET 24000 INTO THE MODE CTL PANEL ALT WINDOW. VNAV AND LNAV WERE ENGAGED. EARLIER IN THE FLT I HAD HAD DIFFICULTY IN ENTERING THE XING RESTR ON THE LEGS PAGE OF THE COCKPIT DISPLAY UNIT. WHEN THE 24000 OR 240 WAS MOVED FROM THE SCRATCHPAD TO THE DESIRED ENTRY POS; AN 'INVALID ENTRY' MESSAGE WAS DISPLAYED. I TRIED THIS ENTRY 5 OR 6 TIMES BEFORE THE CDU ACCEPTED IT. (I LATER NOTICED THAT THE WAYPOINTS BEYOND RIDGY SHOWED THE CRUISE ALT OF FL370; NOT FL240.) THE ACFT SHOULD HAVE DSNDED TO CROSS RIDGY AT FL240. IT DID NOT; AND I FAILED TO MONITOR THE DSCNT TO OVERRIDE THE FMS. AS WE APCHED RIDGY; CTR ASKED IF WE WOULD MAKE THE XING RESTR. WE WERE 5 MI FROM IT AND STILL AT FL370. THE FMS SHOWED THE TOP OF DSCNT POINT AS 12 MI TO GO. I BEGAN A MANUAL DSCNT AND TOLD CTR WE WOULD NOT MAKE THE RESTR. WE CROSSED RIDGY AT ABOUT 35000 FT. CTR DID NOT APPEAR TOO CONCERNED. WE HAD A FLT MGMNT SYS MALFUNCTION AND FAILED TO BACK IT UP OR MONITOR IT PROPERLY. SUPPLEMENTAL INFORMATION FROM ACN 258925: WE TOOK THE SAME ACFT TO DENVER AND; WHEN I WAS TESTING THE FMC BY PUTTING FL240 IN AT DIFFERENT FIXES; IT POSED THE SAME PROB AS FAR AS NOT ACCEPTING A XING RESTR.