Many small firms grow by serving one client extremely well, and the account eventually produces most of the revenue. The risk this creates is not only that the customer might leave.
Pricing power moves to the other side
A customer who knows they are the majority of a supplier's revenue negotiates from a different position, whether or not anyone says so out loud.
Payment terms drift longer, scope creeps, and requests arrive with an implicit expectation that they will be accommodated. Each concession is small and each is difficult to refuse.
The supplier's alternative is to lose an account that cannot be replaced quickly, so the rational move each time is to accept, and the terms ratchet in one direction.
The firm is shaped around one buyer's requirements
Equipment, staff skills, systems and even opening hours get selected to suit the dominant customer, because that is where the work is.
Those choices are sensible individually and constraining in aggregate. The firm becomes efficient at one kind of work for one kind of buyer.
When the firm later tries to diversify, it discovers that its capability is specific rather than general, and that winning different customers means investing again rather than simply selling harder.
Lenders and buyers price the concentration
Banks assessing a loan look at where the repayments come from. Revenue resting on one contract is treated as more fragile than the same revenue spread across many.
An acquirer applies the same logic and usually harder, because the contract may contain a change-of-control clause that lets the customer walk on completion.
So the concentration reduces access to capital at precisely the moment capital would help diversify, which is why the problem tends to persist once it has formed.
Diversification competes with delivery
Finding new customers takes the attention of the same people who deliver for the existing one, and the existing one pays now.
Firms that escape the pattern usually ringfence capacity for new business rather than treating it as spare-time work, and they accept lower short-term output to do it.
The decision is easier when the dominant account is healthy. Once it is shrinking, the resources needed to replace it are already committed to defending it.
The warning appears in the relationship first
Concentration risk usually becomes visible before revenue moves. Familiar contacts are replaced, review meetings turn formal, and the firm is asked to bid for work it previously received directly.
Each of those suggests either that the buyer is testing the market or that an internal sponsor has moved on, and both tend to precede a change in volume.
Suppliers who read them as ordinary administrative churn give up the months of warning that would have been the most useful part of the signal.