The Richards Group is far from the only agency to lose a client after a long relationship, and it won't be the last. But that doesn't lessen the sting much.

Longtime agency-client relationships end for a number of reasons -- among them new management at the client, slipping sales or a need for new capabilities. But depending on how it's handled, the loss of a marquee account can send shockwaves through an agency, with the potential to damage the shop's reputation or spur the loss of talent.

"One thing agencies should do is understand if you had anything to do with losing the account," said Avi Dan, CEO of Avidan Strategies, noting that an agency should do a realistic assessment after a loss. "Sometimes it's just a matter of a new CMO coming in, but sometimes the agency does contribute to it."

An agency should worry more over a string of losses than one departure, even a big one, Mr. Dan added.

A search consultant who wished to remain anonymous said The Richards Group "is more than just Chick-fil-A," with big clients like Home Depot, but the fast feeder's loss is "more emotional because they really put Chick-fil-A on the map." The consultant added that "20 years in this competitive category is amazing."

The end of a long-term marketer-agency partnership rarely leads to reputational problems because the agency has so many years of great work and expertise in a particular category under its belt, according to this consultant. BBDO and GE have worked together for nearly 100 years, but no one would question BBDO's merit if GE finally parted ways, the consultant said.

Much can depend on what happens after a split. BBDO, as a matter of fact, made a comeback in 2012 when it lured former client Visa back after losing its longstanding Gillette business, Mr. Dan said.

"You don't succeed by avoiding punches," BBDO Global CEO Andrew Robertson told Ad Age earlier that year. "You succeed by being able to pick yourself up."

Agencies can incur layoffs, watch creatives jump ship or lose further clients when one high-profile account goes, said Ark Advisors partner Ken Robinson. But in other cases, agencies don't feel any shockwaves after a loss.

That was the case with The Martin Agency and Walmart after nine years, TM and American Airlines after 34 and Haworth Marketing & Media and Target after 46, Mr. Robinson said. Ad Age reported in April that Haworth was in talks to pick up media duties on Walmart's account instead. Looking to add an account in the same category as a departed client is common practice for agencies, according to Mr. Dan.

One key piece of advice for shops that suffer a loss after a long time: Be gracious, no matter who is at fault or how much the shop did to build the client's business. "It doesn't serve an agency well to get into a food fight with a client that has just fired you," Mr. Dan said. "Acknowledge the fact that the client was great to work with and celebrate the heritage and the great work."