Sometimes the puzzles are unsolvable (all the pieces are in place, except for the last 2 pieces that are inverted). Please make sure to generate a solvable puzzle when you randomize the positions of the pieces. You can find more info about this here: http://www.math.ubc.ca/~cass/courses/m308-02b/projects/grant/fifteen.html...

unsolvable configuration I got https://i.stack.imgur.com/e5PRG.jpg solved, but didn't register https://i.stack.imgur.com/ZtkCr.jpg