Was it possible that the lightning bolt symbol on Ash's Pikachu's Poké Ball was made out of paint that easily peels off in water or the writers/developers forgot to put it in there in other episodes aside from the first? We only got to see it in the very first episode and in none of the other episodes involving Pikachu's Poké Ball showed the lightning bolt symbol. During the first episode, Ash went swimming in his clothes and ran through a rain storm while getting Pikachu to safety, which is a possible sign that the lightning bolt symbol was peeling off the Poké Ball as it wasn't shown.