There is always the risk that clients move away when conversations and the level of engagement fades or key members of your team move away.
You fail to leverage the true potential within the client asset because your business development or marketing teams are talking to the wrong people within your client's professional services firm.
A missed cross-sell opportunity is your competitions open door.
If this happens for clients, it happens for referrers too. Feeling under-served they too move away.The cost mounts and will be exposed when the life-time value of a client leaves the revenue stream. Weighed against the cost of acquiring new clients the loss accumulates.