It all really depends on the game that you are playing - your idea of playing with it might not be an effective method in increasing your Pokemon's happiness. Look at this page and go do the stuff that is listed under the game you are playing. There are some methods that do not affect the happiness of a Pokemon at all - one of the most disputed ones are Pokemon Amie/Refresh in X and Y/Sun and Moon. A Pokemon's Affection stat in Amie/Refresh is completely different from its happiness stat in the main game!
Regardless, your Pichu must be close to evolving. That Soothe Bell is only doing him favours - I also assume that you're walking around with him a lot. Just be patient, it'll evolve soon! If you want an indication as to how close it is, go to the happiness checker in your game (also listed in the above link). Remember that Pokemon only need a happiness stat of 220 in order to evolve. Good luck!
Hope I helped. :)