Hidden quest not activating

It must be really frustrating when you've cleared up to level 6 and completed all the level 5 puzzles, but the hidden reward isn't showing up. I had a similar error happen to me before, so I totally get how you feel. Sometimes this can be a temporary issue caused by a delayed server update. I'd recommend trying to completely close the game and reopen it, or log out and log back in. Restarting your phone itself can also surprisingly fix things often. If it still doesn't appear after that, it's likely a data corruption or bug within the game. It would be even better if you have screenshots. You should report the error through the 'Contact Customer Support' option, usually found in the top-right menu or settings. If you specify which levels you've completed, they usually check it and manually grant the reward or patch it. I hope it gets resolved quickly so you can receive your reward!