When I got my butterfly bush I went through the same thing. I kept it in the west window and kept it watered...then I put it outside still in its pot and it started to falter. So I brought in back inside, and later on it began to spring back to life...whew.
It's now outside, this is its second summer outside and it's doing great.
Only time will tell. They are pretty tough buggars.

I'll just wait & see what happens, thanks!

It's been so hot lately libby. I think it would be a good idea to shade it temporarily until it looks healthier. You can break leafy tree branches off and stick them in the ground around it, or put a lawn chair over it if it's not too tall. Then keep it watered the rest of the summer. They do like a lot of sun. You just have to pamper things that you plant this late. It will do fine after its roots have settled in.
With a butterfly bush, I don't think it will matter that you didn't amend the soil.

We moved from the SW to a rental home & expect our household goods soon (thought it'd be the 11th, but was told they left the 11th & would arrive the 21st, but was told they left the 18th & tomorrow/the 24th will arrive). So I can look for something to put in front of it then, & then start playing in the dirt for real!

CHEERS!

Oh yeah, to get back on track, as an afterthought, I hope where I planted it isn't just dirt over concrete. It was very hard to dig. It's in an inside foundation corner.

What eats butterfly bushes? Something is really making ribbons out of my bush.

Earwigs can be a problem with Buddleia. They usually come out at night, so go out with a flashlight and see if you find anything. You can make earwig traps if they are the culprit.

there is a moth which will eat butterfly bush.

If you have a failing plant, take cuttings and restart it.

Nothing lives forever.