I was given a 2 foot tall butterfly bush
that came from a nursery. It had
lots of beautiful purple blooms on it.
Now when it blooms the flowers are
alot shorter then the original blooms.
What is causing this change?

How long have yyo had it.?
In a pot or in ground?
Water, fertilizer, sun?
I cut off dried up flowers during the summer, not sure if leaving them on reduces size of later bloom spikes. The branch makes new blooms from new side branches, probably a little smaller than the initial tip bloom.