Bupa asked WCRS to foot the bill, which is understood to be around £80,000, and the agency is asking Bupa's other marketing services agencies to contribute.
The poster campaign, part of the corporate activity that included the "get well sooner" and "the patient will see you now doctor" executions, involved running two versions of a poster execution.
One was intended to run on a large number of sites, with the other execution running on just a handful. However, the reverse happened and the wrong execution ran on the majority of sites.
Bupa's incumbent media agency is Starcom Mediavest, while Partners Andrews Aldridge works on its direct marketing and Poster Publicity handles outdoor planning and buying. It is understood that those approached have refused to give any money.
The campaign launched in April this year but the payment issue has yet to be resolved. WCRS has held the Bupa account for the past five years, after snatching it from Ogilvy & Mather in 1999.
This year, WCRS has also created a range of television ads, altering Bupa's positioning to one of a more friendly and approachable organisation.
The ads feature cartoon characters and use the line: "Bupa, the health and care people."
A Bupa spokeswoman said: "The poster campaign was a success and as far as we are concerned, all bills have been paid." WCRS refused to comment on the story.