Re: Crown sepatates from ceiling
Wood expands and contracts due to humidity, primarily across the grain or width. During the heating season the air is hot and dry and the wood shrinks. during the off heating season the humidity goes up and the wood expands. This can be minimized if the wood is finished on all sides. If you don't want to take the crown down to seal the back and ends you should be able to fill the cracks with paintable caulk to help hid the gaps and the caulk is flexible enough to move with the wood.
Be sure you live your life, because you are a long time dead.-Scottish Proverb