Greetings David:
I would give these several "late bloomers" a chance before you prune or coppice. Chances are these trees are from a different provenance or seed zone, and thus are physiologically adapted to break dormancy later than your other trees. If they have broken dormancy, then they are viable; however, it doesn't mean that you're guaranteed 100% survival. I've seen many planted trees break dormancy only to die later in the growing season...that's just nature.
We have some white oak species here on campus that have just now broken dormancy...