SEO competitive landscape analysis

Our online retail business was created in 2003. We were, for many years, the industry leader in our space and leader with SEO. We have a solid, reputable business that has a national reach and served many happy customers for many years. We have always adhered to Google's policies and "played by the rules." Our business is critically dependent upon top placement in Google search results for certain terms. We have, over the past 3 years, fallen down the Google search rankings despite sustained investment in SEO with multiple SEO optimization agencies. We are not competing against sophisticated, Fortune 500 companies. We struggle daily to understand what our competitors are doing better than we are, especially given that we have (in our best judgement) tended to investment in SEO, and build a strong reputable business, and have the most longevity of anyone. Recently, we re-launched a rebuild website and engaged a 4th different SEO partner. Each partner has given us a different set of advice, which we have tried to little effect each time. This latest effort is potentially our last best hope. I am looking for help in understanding what methods our competitors are using that have caused us to fall from the top of Google's search results. We lived for many years as the #1 or #2 result for certain search terms, and now, for the past 3 years, have fallen to at best 7th or 8th despite continued investment. I would like help in understanding how our competitors have bested us in this regard. I am not looking at this point for a critique/analysis of our site, as we have already spent considerably in that regard. Rather, I would like to identify what our competitors--many of whom just joined the business in the past 5 years--are doing better than we are.