Well I´ve been having problems with following some flightplans, but the problem is just when I have to intercept a dme arc on SID, the plane follows every fix before the arc starts but when the arc begins the plane just go straight.

i check avionics and they are on,
FMC configurated
AIRAC 1007

so the routes i have been having trouble are (dont remember all of them):

1)MMMY-MMMX
    -RWY 11
    -RTE: MTY  URSUS1A UJ81 TUMAL1A MEX

2)MMAA-MMMX
    -RWY 28
    -RTE: ACA VELAS1B UJ21W TEQ J21 MEX


Does anybody know why??? is this just happen to me??,
Does anybody know how to fix this??

well I´ll wait for your help!
Greetings!