Of course the state is being withholding on details, but there’s a few things that could have happened, possibly simultaneously (Note: my source is listening to NPR):

1. The injections took an awfully long time compared to normal. The medical staff performing the injections have been accused of not being properly trained/qualified. Perhaps not all of it was actually injected into the veins? Witnesses reported that she was singing after the first injection.
2. The older, more reliable sources of the drug used (phenolbarbital) now refuse to sell it to Tennessee prisons. The source of the drug may have been unreliable. Perhaps it wasn’t as potent as advertised.
3. The drug can apparently expire/get old, reducing the efficacy to some degree. There is no guarantee they had a fresh supply.

It’s worth noting that as of yesterday, she was reported as being on a ventilator at the hospital.

Also I just wanna add that the wrestling character in the picture is (I believe) The Undertaker. The actor is now the mayor of Knoxville, Tennessee. This is a very Tennessee post.