At the time, executives were confident that the deal would close as soon as December – just five months. But the hold-up in winning tax approvals as well as obtaining antitrust clearance from China and working through issues with the US Securities and Exchange Commission have delayed that timeline.
Brian Wieser, an analyst with Pivotal Research, asked: "Can we say we have 100 per cent confidence that they are going to find a way to do a deal? No.
"To the extent that these issues were understood at the time of the merger and they assumed they wouldn't be problematic, clearly that was mistaken."
The failure to close the deal has also spooked some investors who fear the transaction may fall apart.
Read MoreThe $35 billion Publicis-Omnicom merger 'started as a joke'
One investor who recently sold its position in Omnicom said that the deal was "a culture clash riddled with tax problems" and suggested that the chance of it completing was being viewed by many investors as only 50 per cent.
In the meantime, Publicis and Omnicom continue to operate separately, and their two bosses insist that they will be fine if they go it alone. In the first quarter of 2014, Publicis reported a 3.3 per cent increase in organic global revenue and Omnicom reported a 4.3 per cent increase in organic global revenue, compared to the same year-earlier period.
"We believe we are very well-positioned to compete in an increasingly complex dynamic landscape," said Mr Wren.
Mr Levy said last week: "That process will go to its end, and we will end up with a merger, a merger of equals that will give a formidable new player in the industry. And if by accident things don't happen, life is good for Publicis. Life is good whatever happens."
Follow us on Twitter: @CNBCWorld