A COURSE DEV ON THE TRAGR RNAV 2 ARR TO LAS; NV.

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DEV FROM CLRED ROUTING. WE WERE ON THE TRAGR RNAV 2 ARR INTO LAS WITH THE AUTOPLT ENGAGED. SOMETIME AFTER TRAGR; THE CTLR ASKED US IF WE WERE GOING DIRECT SHAND WHICH IS A FIX ON THE ILS RWY 25L APCH. WE LOOKED AT THE FMS AND REALIZED THAT WE WERE; EVEN THOUGH WE SHOULD HAVE BEEN GOING TO IPUMY. THE CTLR'S RESPONSE CAUSED ME TO LOOK AT THE CAPT AND SAY 'IT SOUNDS LIKE THIS HAS HAPPENED BEFORE.' THE CTLR THEN SAID THAT IPUMY HAS BEEN 'DROPPING OUT' FOR US (COMPANY). AFTER A VECTOR AND THEN PROCEEDING DIRECT TO BLD; WE LANDED UNEVENTFULLY. WE HAD NO TFC CONFLICTS FROM OUR ERROR. THE CAPT CALLED ATC WHEN WE LANDED AND WAS TOLD THAT WE WERE THE 7TH COMPANY CREW TO DO THIS SAME THING TODAY AND THAT THEY HAD NOTIFIED OUR COMPANY ABOUT THE PROB. ATC SAID THEY WERE REALLY JUST TRYING TO FIND OUT THE FACTS OF WHAT WAS GOING ON AND DIDN'T SEEM TOO BOTHERED ABOUT OUR DEV. I AM NOT 100% CONVINCED THAT THE LAST 3 FIXES ON THE ARR WERE NOT AT SOME TIME IN THE FMS AND THAT THEY DROPPED OUT. I SAY THIS BECAUSE WHILE IN CRUISE I BEGAN TO VERIFY THAT THE ALT AND SPD XING RESTRS FOR THE ARR WERE ENTERED IN THE LEGS PAGE. THE CAPT REMEMBERED SEEING ME CHK THEM. ALL 3 FIXES THAT WE MISSED HAVE ALT OR SPD RESTRS. HOWEVER; I CANNOT RECALL SPECIFICALLY VERIFYING EACH AND EVERY ONE. IF I DID NOT; IT WAS DUE TO A DISTR BECAUSE MY NORMAL ROUTINE WOULD HAVE HAD ME CHK ALL OF THEM AND I WOULD HAVE; THEREFORE; NOTICED THESE FIXES WERE NOT IN THE FMS. REGARDLESS; I SHOULD HAVE MONITORED THE FMS MORE CLOSELY AND VERIFIED AFTER XING EACH FIX THAT WE WERE PROCEEDING TO THE NEXT ONE. AS A SIDE NOTE; IF THE CTLR HAD LET US DSND VIA THE PUBLISHED ARR INSTEAD OF ISSUING US ALTS AND SPDS HIMSELF WE WOULD HAVE NOTICED IPUMY WAS NOT IN THE FMS. ATC SEEMS TO DO THIS REGULARLY. THEIR RESTRS ARE USUALLY VERY CLOSE TO WHAT IS ON THE PUBLISHED STAR SO WHY NOT JUST LET US FLY WHAT IS PUBLISHED? CAREFUL MONITORING OF OUR PROGRESS ON THE ARR WOULD HAVE PREVENTED THIS FROM HAPPENING AND I WILL BE MORE VIGILANT FROM NOW ON.