just becuse you have done that,doesnt mean that it will evolve immediately. feed it more vitamins,heal it with potions alot when I gets hurt.
on hg you can give it massages in the battle tunnel in goldenrod city,and in pearl there is an area post game where you gan get a massage once again,also in veilstone.
dont feed it bitter food,and level it up alot.
and one more thing... budew evolves by happiness but only in the daytime