Unsuccessful SEO campaigns and lack of results

Unsuccessful SEO campaigns and lack of results

We are all talking about how to improve SEO for websites, we have plenty of advises and proposals for our clients, especially at the begging of the "contract".

Most of us offer a wide range of SEO services claiming various results and a successful rise in sales. But what happens when we don't deliver on everything we promised?

SEO is not an exact science, it takes a lot of trial and error type of work and some websites and extremely difficult to rank for various reasons. So what happens when the client paid you for 6 months of SEO implementation and the results are not visible, the websites gets the same amount of sales?

Have you ever run into this type of problem? How do you deal with the client?

I feel blessed because all of my long term SEO campaigns had resulted in direct results and improvements in traffic and ranks, but I'm scared about the fact that someday, my luck will run out and I'll get a website that I simply can't improve and I will not be able to figure out the reason behind it.
What am I going to tell the client?

Please share your experiences on this subject, I love reading success stories, but sometimes to avoid future problems you need to learn from other people's mistakes!