The ceiling was a bit wavy in our kitchen and there were some gaps (some an inch big) between the crown and the ceiling. I first filled in the huge gaps with joint compound, sanded smooth and then painted it the ceiling color. That worked great for a short time, then a crack developed. I researched a caulk that was the very crack resistant,DAP Dynaflex 230. That lasted for a while until just recently when the weather changed. Now there's cracks along the whole run. I could just add more Dynaflex caulk, or is there something else I could use that would keep me from having to do this every year?

 

Welcome to the forums!

That's an issue I know well It's a two fold problem, partly caused by the humidity changes throughout the year and partly by modern construction methods. Polyurethane caulks seem to work the best but might be hard to find if you live in a small town like me.

Ideally the crown molding would be nailed tighter to the framing but often there isn't any wood to attach it too at the ceiling

 

There should have been a backer strip installed before the crown went up so there's something to attach it to at the top.
crown molding backer strip - Bing Images