I play the medium skill level. Probably every other time I play, it results in an unsolvable puzzle. My wife, a long time player of the sudoku premium, concurs and is quite surprised since her app always works OK on her tablet. We both use samsung s2 tablets.
If you come across a puzzle you deem unsolvable, please take a screenshot and post it here. Then use the "Undo" function to take the puzzle back to its original condition and take another screenshot and post it here. We will then be able to investigate.