Hi,
Have you submitted you sitemap to google webmaster account?
Do you change you title time to time according to your main keywords?
Do you change your site content time to time?
Do your site is well optimized?
Did you check you competitors sites and compare your site with his profile?
Do you concentrate yourself for quality link building with your anchor text?
There are several things which can effects you SERPs, now think about your work and do accordingly.
According to me you should check your competitors profile and work according to competition, i am sure your positions will be better than other competitive sites.
Thanks 
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