I recently installed your sudoku game on my Android phone and playing in Extreme mode, I have been presented several times with unsolvable puzles.
Not good. I had it installed previously and this didn't happen. What changed?
Thank you.
I have been playing for years and have never found an unsolvable puzzle.
If you find a position where you think there are no more deductions to be made, just ask for a hint or two.
If you find a position that you think could have two solutions, record the position on paper. Make two copies and try both possibilities on paper. I have always found that only on works.
I don't believe, based on extensive experience, that Genina generates unsolvable Sudoku puzzles.