If you fully EV trained them and maxed out 508 EVs, thier EVs are always there. EVs do not disappear unless manually reset via berries or through some NPC methods (like Join Avenue Salon/ Cafe).
EVs are permanent. You can a fully EV trained level 5 Pokemon (as some people do for Little Cup) and it will always have all those EVs all the way up to Level 100.
The most likely reason you feel your Pokemon are "weak" is because of their IVs. If they have low IVs, they will not be able to compete with a Pokemon that has higher IVs and is also correctly EV trained. EV training is only part of the equation to making a Pokemon stronger. The other most common method is cultivating good IVs through breeding.