We can't really tell from afar why it is having some trouble leafing out completely. Sycamore trees are valuable and tough native plants though so we wouldn't worry too much. They regularly deal with a disease (which is also native) and it doesn't set them back much.
The best thing you can do for your tree is to reduce stress where possible. We would give the tree a ring of mulch out to near the dripline (width) of the branches,  which will reduce the competition for nutrients and water from your lawn, keep the roots more cool and moist and also offer some protection from mowers and string trimmers.
Mulch should be no deeper than 2-3 inches and kept pulled back from contact with the trunk.
Here is our mulching information: 
https://extension.umd.edu/resource/mulching-trees-and-shrubs/


Christine