VERY IMPORTANT—Stat gain from EVs is NOT added up all at once, but gradually as your Pokemon gains levels. At LV 100, your Pokemon will have all the stat gain from EVs it is supposed to, but you may not see large stat gains all at once. For example, if you have a low level Pokemon battle 20 Zubat, you will not necessarily see a +5 gain when it levels up. However, you probably WILL see this kind of gain in a LV 50 Pokemon that’s just starting to battle.
Here's another example, since people still seem confused on this point:
Since EV-training adds +63 in a fully trained stat TOTAL, you're not going to see huge leaps and bounds of stats every level. People new to EV-training expect to see +10 in the EV-trained stat every level, but that just doesn't happen since the max is 63. EV-training still makes a difference, but it doesn't add hundreds of points to the stat. Seeing only a +1 or +2 to a stat is NORMAL, and doesn't mean EVs aren't working.