Certified GKH Gardening Expert
Is the tree suffering from diseases or pest infestation.
Has the weather effected the health of your tree; winter damage, excess rain, drought conditions?
Are other trees in the region leafed out?
Is the soil well draining?
This link may help you access the care needed.
You may also want to talk with your County Extension Office to see if other trees in the area are having leafing issues.