It depends. See the "Feel" stat of the pokemblock? The higher that is, the less the Pokemon will be able to eat.
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This happens. If you gave your pokemon alot of pokeblocks, it will not eat anymore. Which means; It's done forever. You will never be able to give your pokemon a pokeblock.
There is a limit to how many PokeBlocks a certain Pokemon can eat before you can't give it any more, so use PokeBlocks wisely.
Pokemon can only have a set number of pokeblocks. Once they have eaten that set number, they will never be able to eat any more. The only way that you can get your pokemon all the way to the highest amount of a skill is to make really good pokeblocks.
There are no PokeBlocks in HG/SS.
Train the Pokemon you may not know it but it helps the Pokemon win contests if the Pokemon likes you.