Originally Posted by
PrincessCupOButter
Please check in your blue goalbook (top left on your board). Look at the goal titled “Dearest Diary”. That one piece that is not checked is the one you are missing and there is a hint telling you where to look for that piece. If you are missing the diary lock, it is supposed to be found at the hatchery, but multiple players have reported an issue with this. It seems that only a few low level hatcheries, for a very few players, have produced the lock. SpiritWind has reported the issue and hopefully a fix will be in soon.
If you already have the lock, then just follow the hints in the goalbook to whichever location your missing piece can be found.