Why Am I Stuck in Sudoku? The 5 Most Common Causes

You've scanned every row, column, and box. Nothing moves. The puzzle feels impossible. Before assuming you've hit a wall, check these five causes — one of them is almost certainly the issue.

1. Your candidate list is incomplete

This is the cause about 60% of the time. If you've been solving by eye instead of writing in all candidates, you've missed eliminations. A single candidate you didn't notice can block ten other cells.

Fix: Erase and restart candidate markup from scratch. Go cell by cell, row by row. It feels tedious, but it's faster than staring at an incomplete picture.

2. You've been scanning rows and columns but not boxes

Hidden singles appear in boxes as often as in lines. When you scan a row for digit 7 and find two options, it doesn't help. But scanning box 5 for digit 7 might show only one valid cell.

Fix: For each unsolved box, cycle through every missing digit and ask: does this digit have only one valid cell in this box? Do this for all nine boxes before looking at lines.

3. You haven't looked for naked pairs

When naked singles and hidden singles run out, naked pairs are usually next. Two cells in a unit with the exact same two candidates lock those digits and eliminate them from the rest of the unit.

Fix: Scan for cells with exactly two candidates. Check if another cell in the same row, column, or box shares the identical pair. Naked pairs explained →

4. You made an error earlier

If you're genuinely stuck after correct technique, there's a decent chance a digit was placed wrong earlier. One wrong placement creates contradictions that block an increasing number of cells over time.

Fix: Check every placed digit against its row, column, and box. If you find a conflict (two 7s in the same row), backtrack to where that digit was placed and reconsider it.

5. The puzzle requires a technique you haven't used yet

Some puzzles genuinely need pointing pairs, hidden pairs, or X-Wing before they open up. If you've checked for all the above and the puzzle is still locked, the next step is methodically applying each technique once.