I have won all races at both Wellspring and Subway town but did not get the xbox achievement!! Is there something ive missed?

Are you sure you got them all? I think there's ~27 races/rallies/timetrials total

Thanks, I won the 9 available in wellsprings andhave won the available 6 in subway town. How do I access more?

you should be able to scroll right over to the next set of races in subway town. They are locked out at first, but if did the first set then you should enough certificates to do the next set

Yh there's 3 sets in wellspring and 2 sets in subway town.