So I have a ton of eggs in storage and some of them are duplicates. My process is to hatch & evolve the duplicates in order to trade for Essence. More and more I am finding that I cannot trade certain dragons after evolving them to level 4. So far this has happened with:

Begonia (Ultra Rare)
Deep Space (Super Rare)
Fantasy (Ultra Rare)
Neo Forest (Super Rare)
Peony (Super Rare)
Red Giant (Super Rare)
Starfall (Ultra Rare)

I can't think of any good reason why these dragons cannot be traded. Any insight would be appreciated. Thanks.