Failure to strike a deal with such a like-minded country as Canada would call into question the EU's ability to forge other deals and damage credibility already battered by Britain's vote to leave the bloc and disputes over the migration crisis.
The moves came as leaders began a two-day EU summit in Brussels with trade policy the main topic set for Friday. European Council President Donald Tusk, chairing the summit, said in a tweet that the Europe's credibility was at stake.
Wallonia is home to some 3.5 million people, less than 1 percent of the 507 million Europeans CETA would affect.
The European Commission, which negotiates trade deals for the bloc, offered new concessions in the form of changes to an EU declaration to be appended to the treaty. One EU diplomat said these sought to answer Walloon concerns about farming and how trade disputes with Canadian companies would be settled.
Walloon premier Paul Magnette called an emergency session of his government and was due to address the Walloon parliament on Friday. The government recognised improvements, but believed they did not go far enough, an official said, adding it needed time to reflect.
"At this stage, for us, the document is not sufficient," Magnette told reporters in the regional capital Namur.
Magnette said he hoped to meet Canadian Trade Minister Chrystia Freeland again on Friday. The two met on Wednesday.
"We'll see how we can modify the text that we were given," he added.
At the summit in Brussels, Commission President Jean-Claude Juncker said: "I am still hopeful that we will come to a good result in the course of the night and tomorrow morning."