Hi Guy
Whilst you have said you are using all new inbound marketing practices, it's a difficult question to answer with limited knowledge of the details of what you are doing for an eCommerce site you have not said what it sells. You may be doing some of the inbound things well but others not so well.
You've heard it before I am sure, but high quality unique content is very important - even for an eCommerce site, Does the site have a blog? Have you written articles about how to best use the products the site is selling, or how to look after them, or what people are saying about them, or the story behind how the products are created?
In terms of the sites ranking above yours, take a look at what they are doing, Can your site do it better than them?
And what about the site's brand identity? Are there ways you can grow the brand's fan base or a community of followers around the products you are selling.
Depending on the sector your site is supplying to, some or none of the above may apply, but primarily it is important to get inside the head of you customers, understand what questions are they may be asking and shape what you do on the site for them and not for the search engines.
I hope that helps,
Peter