Butterfly bushes should come back from year to year in your area, although sometimes they may die back to the ground during the winter if it is a tough year. Usually, they are well leafed out by the end of May. However, in some cases after a hard winter, they may die back to the ground and can take until about now to start sending up new growth from the roots.
Butterfly bushes are deep rooted and can consequently be difficult to sig up and transplant successfully. I think a little more patience before you give up on it, but I would not be too surprised if it did not survive a transplanting procedure. This is especially true given the relatively hot dry weeks we have just had instead of a cool, rainy April and May.
If on the other hand by transplanting you mean you planted a new plant for the first time, it is possible that it is taking its time in leafing out due to transplant shock. We had some very hot weather and a dry spell that made it difficult for new plants to become established.
The soil should be kept evenly moist but not soggy during the first season, and if it was allowed to dry out it could have killed the plant or set it back somewhat. Again, though, I would allow a little more time before giving up.
Q&A Library Searching Tips