A B737-300 FLC FAILS TO START DSCNT IN TIME TO CROSS THE VOR AT FL280 OVER HEC; CA.

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I WAS FLYING IN CRUISE AT 31000 FT MSL. AT APPROX 65 MI FROM HEC WE WERE GIVEN A XING RESTR OVER HEC OF 28000 FT MSL. I PROGRAMMED THE FMC FOR HEC AT 28000 FT AND THE CAPT PUT 28000 FT IN THE ALT ALERT WINDOW. THE FMC SAID ABOUT 50 MI TO TOP OF DSCNT. A RADIO FREQ CHANGE WAS GIVEN AND DONE. THE CAPTAIN AND I STARTED DISCUSSING A FUEL LOADING PROB ON A PREVIOUS FLT. THE NEXT CALL WE GOT WAS FROM ZLA SAYING 'DID THE LAST CTLR GIVE YOU A XING RESTR?' WE BOTH IMMEDIATELY REALIZED OUR POS OVER HEC AND STARTED AN IMMEDIATE DSCNT. THE CAPT RESPONDED WITH THE RESTR. THE ANSWER WAS 'YOU JUST PASSED HEC.' THE CAPT VOLUNTEERED TO TAKE AN IMMEDIATE TURN OUT OR NEW ALT. NO RESPONSE WAS RECEIVED AND NO MENTION OF IT CAME AGAIN FROM ZLA. OBVIOUSLY; THE CAPTAIN AND I WERE INVOLVED IN OUR FUEL DISCUSSION AND MISSED THE DSCNT POINT. THE FMC GIVES NO WARNING ABOUT A MISSED DSCNT POINT WHEN THE ALT IS IN THE ALT ALERT WINDOW. ADDITIONALLY; VNAV OP IS PROHIBITED AT OUR AIRLINE SO THE AUTOPLT CANNOT INITIATE THE DSCNT WHEN IT IS MISSED. SUPPLEMENTAL INFO FROM ACN 483941: THIS WAS THE LAST LEG OF AN 11+ HR DUTY DAY AND I DIDN'T NOTICE THAT THE FO DID NOT BEGIN DSCNT. I AM FLYING WITH NEW FO'S AND MUST WATCH THEIR EVERY ACTION MORE CLOSELY.