re: How to fix crown molding separating from ceiling?

Too late for nail gun, you need to press in while nailing a no. 6 finishing nail, set with nail punch, remove old caulk and re apply.

In that second pic, it looks like the molding board might have been cut too long and not allowing it to fit tight up against the sheetrock. Before doing any nailing [in the area of the second pic.], try pushing on the molding to see if fits correctly. You might have to take that board off and cut a little bit off the end. I hope this is not the case.