I have a Linden tree. It has buds on it, but they don't seem to be opening up. The buds are on the tree and some are open a little and some are not open at all. It seems stagnant.
The long harsh winter has caused some slow down of trees and gardens across much of the United States this spring.
Being in MN myself I have also had delay's in some on my trees.
Our temperatures have been on the relatively cool side with some inconsistency in evening temperatures.
If the leaves and tree itself looks healthy, I would just give it more time.
It very well may just not preform as well this season due to all of the weather stresses.
You can fertilize if it continues to struggle.
The article below has good information for you.