For really general terms, I'm going to say it's probably your linking strategy.
I'll give you a golden secret here, but this'll help you considerably if you take the time & effort to implement it.
Find out who your top competitors are and analyze them. Their on-page optimization efforts, and their linking strategies.
And type in the URL of the pages of the top 5 or top 10 and look into your competition's inbound links - where they're located on the page, the topical relevance of the site that is linking to your competition and the anchor text used to link to your competition. You can determine if they're paid links (or somehow incentivized, not just cash money) or if they're natural.
Now, to break this down and understand their linking strategies, you'll have to have a good understanding of link building & SEO yourself, but it's a good way to help you determine a linking strategy that will likely help you achieve your goals if you'll follow through with it.
This doesn't work in all cases, and it's not a perfect plan - but it gives you some very valuable information on your competition & what they're doing regarding their link building strategies.
If you need any help, I do offer competition research services - it's not really cheap, but what I do is listed here (albeit in a condensed format).