In my many years with a large, international geotechnical and materials engineering firm, we typically would get around 30 percent proposal success. Then the firm transitioned from an engineering firm run by engineers to an engineering firm in name, run by accountants and marketing types. That's when suddenly 30% was deplorable and the mandates began that all senior and higher level engineers had to put out more proposals, bill more time, do more marketing, etc.....funny thing though...the company's profitability, viability, and return on investment were never as good under the accountants as under the engineers. Firm was eventually sold/merged/pilferred to a hollow shell of its former leadership in the profession. Sad to see. I left before the final demise.
Given my experience, I agree with McGiverS2000. Focus on doing good engineering work and developing personal relationships with clients....you'll get plenty of work.