To improve search engine ranking position (SERP) of a targeted keyword at Google: 1. make sure to provide original & unique content that copyscape 100%. and update content regularly. Due to Panda's algorithm. 2. acquire backlinks. To diversity backlink kinds and also author texts because of Penguin's algorithm. Both content and backlink are vital factors to improve SERP at Google.
It really depends on what red flags you set off. It could just be lack of link diversity. You might need to get links from a wide variety of sources, using a wider variety of keywords. Or they could have determined that you have onpage issues that are unfavorable. I'd compare both your link profile and site quality to competitors that are still ranking well.