One rebel group which agreed a cease-fire with the junta, the Shan State Army (North), also refused to turn up after several ethnic Shan leaders were arrested this month.
Human rights groups and the United States say the arrests show the junta is not interested in democratic reform despite pledges to adhere to a seven-stage "roadmap to democracy" announced in August 2003.
Amnesty International said the arrest of seven Shan leaders on February 8 and 9 sent "an unequivocal message that peaceful political activities will not be tolerated".
The government says, however, it is set on restoring civilian rule after more than four decades of army diktat.
"We are very determined towards the successful implementation of all seven steps of the roadmap," Lieutenant General Thein Sein said in re-opening the convention, the first stage of the army's democracy blueprint for the former Burma.
He exhorted the 1,075 delegates - clad in a colourful array of tribal costumes, feathered head dresses and animal horn helmets - to work hard during the talks, but declined to put a time frame on the roadmap.
"It's very difficult to say how long it will take," he told reporters.
Diplomats say Yangon's ruling generals might try to wrap up the convention in time to proclaim the roadmap on track before they take over the helm of the 10-member Association of South East Asian Nations in the middle of next year.
"I think they are working to their own internal timetable for a constitution, referendum and elections by July 2006, so that they can present themselves proudly to the world," said one Yangon-based diplomat.
However, another diplomat said tension within the junta following the October purge of Prime Minister Khin Nyunt and more possible walkouts by groups which had agreed cease-fires could derail such a deadline.
"Nobody wants to return to conflict, but the government has spent no time looking after the rebel groups and if these guys get nothing out of the process, they are not going to be happy," the diplomat said.
US Secretary of State Condoleezza Rice has pulled no punches, branding the impoverished south-east Asian nation one of six "outposts of tyranny" during her Senate confirmation hearing in January. Washington has imposed stringent sanctions on Myanmar, but its neighbours have preferred a more conciliatory policy of "constructive engagement".
Neither approach appears to have made any difference.
Of the 1,081 delegates invited to the second sitting of the convention, all but 147 have been handpicked by the junta as representing "all walks of life" - from peasants, to workers to intellectuals to ethnic minorities.
According to the official New Light of Myanmar newspaper, 87 percent of them are "educated", the remainder being "wise, well-experienced and respected by local populace". The talks are being held in a specially spruced up military training camp around 40 km (25 miles) north of the capital.
Conditions are "green and pleasant", the New Light of Myanmar says, but with several military checkpoints barring anybody from leaving the red-brick compound, the UN's special human rights envoy has likened it to "mass house arrest".