The EVs are added gradually up to level 100, not all at once. If you got 20 EVs in one level, and you're only level twenty, you will only get a couple extra each level.
If you catch a high level pokemon with no EVs (e.g. those Magikarps in Platinum) then EV train loads of EVs in one level, it will go up a lot more each level.
So just keep training, you get most of them before level 100, but it will always take a few levels after you got them all for them to actually be added to your stats.