In Greek mythology Sisyphus was a king punished by being compelled to roll an immense boulder up a hill, only to watch it roll back down, and to repeat this throughout eternity. wikipedia link
The analogy stems from the fact that I never see the boulder to the top of the mountain: most of my customers invest in the beginning of their transition, to get the boulder going, make sure momentum is given to the initiative. When we are making good progress and achieved some visible benefits, most companies are reluctant to see the benefit of continuing this investment further, to the tipping point to be exact.
Getting a coach in for a long period can appear to be costly, especially if the reasons for going Agile are not clear and well articulated within the company. In some other companies, the investment lasts longer, but the sense of urgency can decay rapidly, leaving the ability to influence and change the said company greatly diminished.
As a Coach, I usually stay with a company between 3 months and maybe 12 months, and it has never felt like the boulder reached the top: oh we got near on many occasions, but eventually we got so near that I had to go (!).
Starting with another client is always fun and full of surprises, it is also starting to push the boulder from the bottom of the mountain again. For me there is nothing to do, after all I have chosen my penitence.
For those companies that want to get the boulder to the top and eventually become Agile, make sure you work with your coach to build some in-house muscle ready to get that baby to the top.