I had a few "issues" with my new 510 batteries after a few days, but I think I created the problem myself. I think I was charging too often, and, when I started vaping until the led flashed, and then putting them back on the charger for at least two hours, those "issues" went away. Also, if its a new kit, and both batteries are being weird, I'd try a different charger.
I'd also like to point out that, no matter how reputable your supplier is, if a product (any product) says its battery will last for eight hours, you can hope for four, as long as you expect two.
John