You may also just be at the owners limit on this one. You achieved great results in a short amount of time and now you are hitting the wall of diminishing returns. This is a good thing as it means you've done your job. However SE rankings is just an abstraction. Why do we all strive for high rankings any? Simple. It dramatically increases our chances of making money. I'd switch your pitch to conversion metrics and ROI. Every business person understands making more money. So when you talk about newsletters, affiliate links and the like, forget about SE ranking and just talk about conversions, customer outreach and retention.