These are some possibilities:
Ralts gained Ev's via the EXP. Share and cannot gain anymore.
If traded, it could already have max Evs.
You can only use Carbos and Calcium for the first 100 Evs, meaning you cannot fully train a Pokemon with them (but they are a faster alternative for the first 100 Evs in most cases).
If not any of the above, provide more information. Hope I helped!