Greetings John,
There are many biological and environmental factors that may or may not be contributing to your tree's sparse crown this spring. For example, this year's late frost (environmental) could be the culprit regarding crown die-back or sparse foliage symptoms. Unfortunately, there is no cure for frost damage.
As I've stated many times throughout this forum, more often than not, only an on-site evaluation by a tree care specialist can provide a legitimate diagnosis for tree health concerns. Even after an on-site evaluation is conducted, further plant tissue analysis may be needed. The University of Illinois Plant Clinic may prove useful in those cases http://plantclinic.cropsci.uiuc.edu/.
Rather than "guess" at your tree's potential ailment based on the information you provided, I would strongly suggest you contact a professional tree care specialist (e.g., a certified arborist from the yellow pages or from the Illinois arborist association website www.illinoisarborist.org/) for an on-site evaluation.
As is often the case, the patient (your tree) and the doctor (tree care specialist) simply need to visit each other :-)
