Role Erosion is a term used to describe circumstances where a power exchange relationship gradually and unintentionally takes on a more vanilla outlook, focusing less on the dominance and submission.
In most cases, circumstances of life such as illness, jobs and family commitments get in the way and prevent a focus on what made the relationship good for the partners in the first place. What was initially a temporary situation forced by circumstance can become a habit (or loss of a habit).
People's lack of willingness to work on the relationship, or their inability to see what is happening to it, is another major factor.
In many ways, the underlying causes of role erosion are very similar to those of the demise any long-term relationship. A good relationship takes work but more than that, it takes careful work with the efforts put in producing the desired result.
If both (or all) partners are happy with the power in the relationship becoming more even then there is no problem, but this is rare.
More commonly, it is only one partner who changes their behaviour to any great extent, while the other partner, instead of doing something about it, keeps quiet rather than discussing the issue. Reasons not to discuss it are varied from the creeping syndrome, where there is no obvious point at which to talk about it because it's been happening so gradually, to a feeling of guilt because the other partner is seen to be working hard in other areas.
With the erosion of what made the relationship once vibrant, doubts, mis-appreciation and dissatisfaction creep in. After a while, these become very hard to heal without a great deal of conscious effort.
Like maintenance with most things, good relationship maintenance is actually more efficient than remedial repairs and leads to greater overall satisfaction. The reason many people don't do it is because it is often not very romantic; doesn't seem urgent (indeed, it shouldn't be); isn't terribly exciting compared to all the kinky sexual practises that is the focus of much BDSM literature.
'Life' can get in the way of the best intentions and circumstances always change over the course of a relationship. A formal, once-a-month meeting to discuss the major relationship factors (money, sex, children, power exchange, work) can go a long way to catching issues before they stand a chance of being a problem.
On top of a monthly relationship health-check, an annual review (perhaps on the anniversary of meeting/reading this/marrying/collaring) going more into depth can help direct the relationship for the future. In a healthy relationship, communication is happening all the time but effort to re-think one's position can be worthwhile. In-depth questions can reveal interesting variations and yet undiscovered desires. The annual review can be very in-depth and even include written essays to help make sure nothing is being left unsaid. Try asking yourself questions like "if my partner died last year and I was now looking for another partner, what would I be looking for?" Even a Master and slave can learn things from reading one another's essays on such a subject.