Here is where 90% of players fail. Limited-time events (Halloween, Christmas, Summer Splash) introduce exclusive puzzles. If you miss these, you can never get 100% completion on your profile.
Many players find that as they approach the final puzzle pieces for a specific character, the difficulty spikes. If you find the puzzles too hard, the developer has noted that they frequently update the game to address difficulty based on player feedback. It is often better to switch focus to a different waifu and return to a difficult level after an update. eroblast all puzzle pieces