Synchronize didn’t work against Sleep Powder because Synchronize does not pass sleep back at all. It only reflects burn, poison, or paralysis, and sleep/freeze are excluded.

Why it happened

If a Pokémon uses Sleep Powder, the target can still fall asleep normally, but the attacker won’t get put to sleep by Synchronize because that ability simply doesn’t trigger for sleep. In other words, the move caused a sleep status, but Synchronize only mirrors a limited set of statuses.

Quick example

  • Opponent uses Sleep Powder.
  • Your Pokémon gets put to sleep.
  • The opponent stays fine, because Synchronize does not reflect sleep.

Important note

Some forum posts mention that Synchronize may appear to “do nothing” in these cases, which matches the mechanic described in the game guides: no mirror effect for sleep, even though the status itself still lands on the target.

TL;DR: Synchronize works for burn, poison, and paralysis — not Sleep Powder.