Hung Up isn't doing THAT bad. At least it's not falling quickly. It's history is Now: 20-21-27-14-7-7-14-19-21-23. If it keeps falling like this it becomes a hit after all. 10 weeks in the top 30 now, not bad. And Hung Up was n1 on the sales chart, so that's a cheer up I hope... If Billboard would be based on sales like other countries Hung Up would have been n1. But that's not the case (and I'm happy it's not, cause Hung Up doesn't deserve that).
BTW, n7 is not a bad peak at all. She made the top 10, so Hung Up was a hit. Just because fans compare her to Mariah in the US, Hung Up seems a flop. But it isn't. A single doesn't have to be n1 to be a hit.

N7 is a nice peak.