I've solved a puzzle but the game doesn't recognize it as solved.

This can happen in the Classic Mode. If all the cells are filled in but the game doesn't recognize the puzzle as solved, this means there are some mistakes in the playing field. You need to find and fix them yourself.

If you want the game to notify you of each mistake, switch to the Standard Mode:

1.  Tap the gear icon in the top right corner.
2. Tap Settings.
3. Tap Game Mode.
4. Select Standard Mode.