It seems a few people have reported this problem, and I'm not sure I understand what's going as I haven't managed to reproduce it.
Here's the general rules:
- If you drop a piece (stop power) it will try to connect with matching pieces AND check if the puzzle is complete (if yes it opens the door)
- Sometimes seemingly "compatible" pieces can stand next to each other without forming a group (if you did a "ground pound" for instance) groups are only formed when you stop grabbing/power (press C again)
- The puzzle can be considered complete even if some pieces are not technically grouped, it just needs to be in the right place.
There you go, not sur if that makes sense or even cover the potential bug you have ;)
Thanks for the feedback anyway it's very useful !!