Why can I only sometimes merge small dragon chests?

My camp is filling up with small dragon chests, mostly harvested from stone but also won at the end of levels. I have them all pushed off to one side while I save up gems to open them.

I've noticed that, occasionally as I'm moving them around, several of the "dragon egg chests" will be merged into a "dragon nest vault", but I cannot seem to make it happen consistently. If there is a pattern to when this does or doesn't work I haven't found it. There's also no progression chart for these items, so I assumed you weren't supposed to be able to merge them, but it happened at least 4 times so far.

What are the rules for when you can merge the dragon egg chests together?

Solution 1:

When you click on those small dragon egg chests, you will notice that the amount of gems you need to open them varies: sometimes it’s 9,11,12, or 17. You can sometimes merge matching ones together, like merging 9’s together, or merging 12’s together. It doesn’t always work, but if you want a higher probability of merging the dragon egg chests, you should clump them together based on the amount of gems it takes to open them. However, after they’ve been merged, you cannot merge any of the dragon nest vaults together, it stops there.