Hi Shabbir
As you've encountered, as you rise up through the rankings it becomes harder and harder to displace those ahead of you.It's a bit like running a marathon starting at the back of the field. You can get to a point where no matter how hard you run, you're never going to catch the guys who started before you.
Ranking for product pages can be insanely tough - even with unique product descriptions...
Brute force link building can only take you so far and increasingly you've got to watch out that your backlink profile isn't going to come back to bite you.
Without knowing what niche you're in it's hard to provide specific advice, but here are some thoughts:
1. You're getting traffic, but how much of these are new visitors and how many are returning visitors? What can you do to get more of those visitors to engage and take action. How can you turn these visitors into loyal returning customers? This can be difficult if you're selling a commodity on price alone.
2. Go broad and think of your long-tail. Instead of focusing on the head-terms that become increasingly difficult to improve, think about growing your long tail by adding content to your site. This can for example, take the form of Frequently asked questions, how-to guides, tips etc or other more creative ideas. It's the richness and depth of thiscontent that'll earn you traffic from queries you would never target individually.
Just remember that these pages still have to work hard to move your visitors towards your site's goal and becoming customers.
Your long tail can deliver more traffic than your head-terms combined. The downside is that the commercial intent may not be as strong for some of this traffic so you'll need to work hard to convert this traffic into customers.
3. Instead of targeting those people who are looking for your products, write content for people who are trying to find answers to the problems that your products solve.
4. Guest blogging just for links doesn't really help you establish your site as an authority and you're missing a trick if your not getting your content in front of your potential customers. Where do your customers hang-out? Can you write guest articles that will attract traffic to your site rather than "just a link". You've got to understand what you're audience is interested in and resist the urge just to tell them what you want to tell them!
Hope this helps,