A B737 FLT CREW FORGETS ABOUT AN ASSIGNED XING RESTR AND HAS TO NOTIFY CTR CTLR THAT THEY ARE UNABLE TO COMPLY.

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ENRTE LAX TO MCO; ZJX ASSIGNED US THE LEESSE NINE ARR WITH THE OTK TRANSITION. WE HAD ALREADY DSNDED FROM FL370 TO FL330 AND DUE TO A BAD RIDE; I REQUESTED TO DSND LOWER. AT THE TIME; MY FO WAS OUT OF THE COCKPIT USING THE LAVATORY (IT WAS HIS LEG) SO I WAS FLYING. I WAS GIVEN A DSCNT TO FL310 AND THEN GIVEN A XING RESTR OF FL270 AT CLAWZ INTXN ON THE LEESE NINE STAR. MY FO RETURNED TO THE COCKPIT AS WE WERE DSNDING TO FL310 AND I INFORMED HIM OF THE CHANGE IN ALT AND THE XING RESTR. HE ACKNOWLEDGED BOTH. WE WERE IN VERT SPD AT THE TIME AND ONCE LEVEL; IT LOOKED TO ME AS IF THE FO WAS SETTING UP A VNAV DSCNT; AS HE HAD DONE THE LAST TIME I FLEW WITH HIM. HE VERIFIED THE ALTS IN THE FMC AS I STARTED LOOKING AT THE APCHS INTO MCO. AS WE APCHED CLAWZ INTXN CTR CALLED AND REMINDED US OF THE XING RESTR. I HADN'T BEEN MONITORING OUR PROGRESS AND DIDN'T NOTICE THAT WE HADN'T BEGUN OUR DSCNT. THE FO HESITATED MOMENTARILY WHEN I TOLD HIM WE NEEDED TO START DOWN IMMEDIATELY; THEN USED VERT SPD TO START DOWN USING SPD BRAKES. CTR COMMENTED THAT WE APPEARED TO BE AT CLAWZ AND WELL ABOVE FL270. WE WERE ACTUALLY DSNDING AND STILL A FEW MI W AT THAT POINT. I TOLD CTR THAT WE WOULD NOT MAKE THE RESTR AND ASKED FOR RELIEF; WHICH WE GOT ALONG WITH A BRIEF HEADING CHANGE UNTIL WE WERE LEVELED OFF. WE LEVELED OFF AT FL270 LESS THAN 1 MIN AFTER PASSING CLAWZ. ONCE LEVEL; MY FO TOLD ME THAT HE HADN'T KNOWN ABOUT THE RESTR. I TOLD HIM THAT HE'D ACKNOWLEDGED ME WHEN I GAVE IT TO HIM; AND THAT I'D SEEN HIM VERIFYING THE XING ALT THAT HAD BEEN SET IN THE FMC; SO I BELIEVED HE WAS FULLY AWARE OF IT. THE ERROR WE BOTH MADE WAS ONCE LEVEL AT FL310; WE FAILED TO RESET THE ALT SELECT WINDOW TO FL270; SO THE VNAV NEVER BEGAN THE DSCNT. WHILE I BELIEVE THE AUTOPLT WAS RETURNED TO VNAV AFTER LEVELOFF AT FL310; I'M NOT CERTAIN OF THAT FACT. HAD I VERIFIED THAT THOSE 2 THINGS HAD BEEN DONE; I BELIEVE VNAV WOULD HAVE OPERATED AS IT HAD BEEN PROGRAMMED TO DO. ULTIMATELY; IT WAS MY FAULT FOR NOT MONITORING THE DSCNT AND NOT VERIFYING THAT THE FO HAD COMPLETELY SET UP THE FMC AND MCP FOR A VNAV DSCNT.