Branded Keyword page harder to rank?

Hi all,

A couple of years ago I create my first site which is still doing quite well. What I did was take a generic keyword that people search (for instance running shoes), and then created a page for various brands (such as Nike running shoes). I found this used to work quite well and was able to rank these branded keywords in the first couple of positions as the others did not have many backlinks.

Lately though I am finding that I am consistently ranking in the #4-#5 spots for branded keywords despite using similar linking strategies. I understand losing to the website belonging to the product (nike for example) - to me this means Google is doing it's job. But i'm a bit confused about why I am losing to other online retailers who do not have any backlinks and have poor SEO.

I have never had an issue ranking about Amazon but am finding that these other large online retailers are very difficult to outrank now. I'm not sure why, is it that they participate in Adwords campaigns? Is it harder to rank branded keywords since all of these Panda updates?

Thanks all!