If you mean the Wings/Vitamins/etc., the other answers are missing a few things.
Basically, the Vitamins increase a Pokemon's EVs by 10 in the desired stat; and a single Pokemon may have a total of 512 EVs, however one stat may only have 255 EVs, to avoid a Pokemon becoming overpowered. For a level 100, every 4 EVs a Pokemon receives is the equivalent to 1 Stat point in the desired stat. Since 255 is not divisible by 4, the highest amount of effective EVs for a stat is 252, the equivalent to 63 stat points. However, this '4' number is increased the lower you go in levels, and the requirements for 1 Stat point can go as high as 400 for a level 1.
However, the Vitamins only work until the first 100 EVs in a stat for whatever reason; this may be a cause. So if you are getting the message, '
It would have no effect.', that means the Vitamins won't work anymore, unless you reduce the EVs using certain Berries or some stores at Join Avenue. The Wings are alternatives for the Vitamins that will still work until the 255th EV; however, they only increase the stat's EVs by 1, which means increasing stats using them is a long process.
So I conclude with: the items are working, but the amount of EVs the items are giving you isn't enough to get a stat point. If you use a few more, you may get a result. To find out how many EVs you need to get an increase for Emboar's current level, try this equation:
400 / <Emboar's level here>
The number in front of the decimal is how many you need. So for level 51, you need 7 EVs.
Source: Experience, + http://pokemondb.net/pokebase/102386/effort-values-at-different-levels