Starting but not finishing

Some employees amaze me. The girl who wanted my job and still expects to get promoted at some point is constantly doing things that show me she is nowhere near ready to be a manager. She is sort-of a manager now, but not really. How to describe it? Occasionally she'll be the closing manager, but if a regular manager is in the building then she isn't supposed to do any manager stuff (like returns or getting change from the safe). She has a major problem with my authority and has an attitude whenever I tell her to do something. She thinks I'm "second-guessing her" or "treating her like an idiot" if I make sure she's doing things right. If I question her taking too much time to complete a task, she whines about how busy the store was and how she was trying her best and I'm expecting too much.

Today she started four tasks and didn't finish any of them.

1. Despite having a shipment of stuff to put away, she started cleaning/organizing an employees-only area, then it must have struck her that we had the shipment to put away so she stopped cleaning. For the rest of the day the stuff was on the floor, ready to trip everyone who walked by. Mid-afternoon someone picked up all the stuff and threw it back where it belonged. So the stuff actually ended up messier than when she began, besides threatening a few lives.

2. She put away a little of the shipment, stopping to re-tie her shoes and complain about how hot it was and how much her back hurt.

3. In the middle of putting away the shipment she stopped to do some routine tasks that could have waited until later. In the meantime we finished the shipment without her. She came back, commented on the shipment being finished, "too bad I missed it, ha, ha," {flippant toss of her hair}. Then she went on break and conveniently forgot about finishing the other routine duties, leaving them for the closing crew.

4. For her last hour at work I asked her to finish putting away some boxes that the evening shift hadn't done the day before. The hour should have been plenty of time for her to finish (it would have taken me about 30 minutes, but I had other things to do). HOWEVER......while she was putting the boxes away she decided that the shelf was dirty so she'd take EVERYTHING off of the shelf to clean it before putting away the boxes. Well, guess what? By the time her shift ended the boxes were only half put away, and someone else had to finish that task, too.

Ready to be a manager? I don't think so. You've got to be able to prioritize and maintain a workplace free of hazards and FINISH THINGS.