Did Ash forget that Pikachu can use Volt Tackle? This is really disappointing. XY ended, but Pikachu did not use Volt Tackle a single time in the whole series, not even once. Did Ash start using Electro Ball so often that he forget about such an important move of Pikachu?
According to some users in this thread, Ash's Pikachu forgot Volt Tackle to learn Electro Ball, which makes sense, as the series seems to follow the games with Pokemon only knowing 4 moves at a time.


  Because they just had to have him learn a Gen V move. He forgot it so he could learn Electro Ball.


According to the wikia page for Ash's Pikachu, this checks out.

The last instance of Pikachu using Volt Tackle was in BW013 (Minccino - Neat and Tidy!).


  As Pikachu headed forwards he got hit by Pignite's Take Down, followed up by a powerful Heat Crash. Pikachu managed to get up and countered Pignite's Flame Charge with Volt Tackle.


The first instance of Pikachu using Electro Ball is in BW020 (Dancing with the Ducklett Trio!). The synopsis for this episode even states:


  After unintentionally shocking himself with Thunderbolt, because he was stuck in a beach umbrella, Pikachu couldn't use Electric-type attacks. Eventually his anger caused him to learn Electro Ball.


After this point, it appears that Pikachu no longer knows Volt Tackle.