Playing level 839 I noticed that after completing level 837 en 839 the giftboxes do not open. How is this possible and how can this be fixed? I'm playing this game on my phone.

Hey both... just sneaking in. The reason the boxes are not opening is because somewhere there is a level the game thinks you haven’t completed. You need to look back and find one without stars. Usually it’s the level the game keeps opening at even if you have passed it, but can be before :)