Vitamins

Well it might be simply because you have already achieved the highest EV level of that Pokemon, you might have to raise the Pokemon level to one hundred if you want it stronger, but I think it might be because you already have achieved the maximum level. If that is not the case in the matter, I wouldn't know what to tell you, you might have done something you shouldn't have like use AR, and it interfered with your game. You might ever have to go buy a new copy of the game. There is definetly something wrong though.

Well it does have an affect but it depends on your pokemons current lvl in that stat. Like say I use a carbos on a lvl 3 pikachu. It wont show that it raised it because it's speed will be so low that it doesnt have much effect. How ever if that pikachu was on lvl 100. It will probably raise it's speed by like 3 or 4, because pikachu's speed will be higher. I hope you get what I meant by that.