Snubull evolved into a Granbull and is gone, which is why it hasn't appeared anymore. The episodes of Spoink had some entertaining moments (with the Magikarp salesman and Team Rocket trying to open Clamperl's shell) and all, but the main plot wasn't as good. The first thing Spoink did in its return episode was bounce up and down until the pearl just flew off and landed into the ocean. I mean, if that's all it takes to lose a pearl, if I were Spoink I would just glue it to my head or destroy it altogether. (Well, maybe not that, but...)