The bud swelling and leaf burst is not a good indicator of the health of trees. This change is chemically induced in response to longer daylight, stronger sun and increased temperature. The true test is if the tree can support this new growth.
Leaf burst should be all over the canopy, if areas are devoid of new leaves or sparse, it is an indicator of stress, the tree can no longer support these areas of it's canopy.
Please do not expect all your trees to come into leaf at the same time. Specimens of the same species may be in colder areas. Trees from warmer climates will be later into leaf.