Missing pieces – Jigsaw Puzzle Plus Review

Just tried to finish the fourth Flower Power puzzle and there are 2 pieces missing. How can this happen? I changed to a dark background to try to locate them, moved the puzzle clear off to the side but they are not there. This is the first time I have encountered this in the many years I've been using this app. Please fix.
Review by hiker-gal on Jigsaw Puzzle Plus.