Time Machine rMBP issues

I'm having some odd reoccurring issues with Time Machine. I have an early 2015 13" rMBP base model that I just got at the end of July. My external HD that I'm using with Time Machine is a Seagate Backup Plus Slim 1TB.

Problem 1-
At night I plug my computer in to charge and I plug in my external HD to backup overnight. I have power nap enabled so that I can close my computer and it will (theoretically) still backup. But when I wake up in the morning and check Time Machine, it says it only performed one backup while it was sleeping and that was after the first hour that I put the computer to sleep. Is this what is supposed to happen? Or is it supposed to backup every hour? Also, am I misunderstanding Power Nap? Does power nap only work when the Macbook lid is open but sleeping?

Problem 2-
Occasionally when the external HD is plugged into the computer and Time Machine begins backing up, it'll get stuck at "0kb of # mb" and won't move any further until I hit "skip backup" and manually click "backup now".

Does this seem like a problem with my external HD? Are these common problems? Any help or advice would be greatly appreciated!

 

Thanks your help is much appreciated!