Same problem on this side of Lake Michigan.
I have a lot of basswoods on my property and up and down the river I live on. The only blossoms I see (which are two weeks early) are branches that are sitting under sugar maple branches. Apparently the already leafed sugar maples protected the basswood buds. I don't see any basswood blossoms, no matter the age or height of the tree, except where protected.
For me it is a big disappointment. Basswood honey is so tasty and usually a major flow for me for 3 weeks in July. Hard to loose that source.