The signing was put off last week after Mr Kony said he was sick
The leader of Uganda's rebel Lord's Resistance Army (LRA) is refusing to sign a final peace deal until some key issues are clarified, officials say.
Joseph Kony was expected to come out of hiding to sign the deal in a ceremony in southern Sudan on Thursday.
But he did not appear. His chief negotiator later resigned, calling the whole process into question.
The Ugandan-LRA conflict has cost tens of thousands of lives and is notorious for atrocities against children.
Two million people have been displaced in the conflict, which has lasted for more than 20 years, and during which the LRA has allegedly abducted thousands of children to serve as child soldiers.
News agencies reported that Mr Kony wanted further reassurances about the mechanisms that would be used to try LRA members for war crimes.
Mr Kony, who has been in hiding since an arrest warrant was issued against him and two of his top commanders by the International Criminal Court (ICC), has been reluctant to emerge from the jungle until those indictments are lifted.
LRA negotiators have already said their fighters will not disarm unless the ICC arrest warrants are removed and they can be tried on war crimes charges in Uganda instead.
Announcing the delay to the signing, South Sudan's Vice-President Riak Machar sounded upbeat, and said he hoped it would still happen before the end of the week.
The LRA are notorious for mutilating victims
But the resignation of rebel chief negotiator David Matsanga threw some doubt on proceedings.
He admitted he had not spoken to Mr Kony in four days.
Dr Matsanga has signed a number of interim deals on behalf of Mr Kony since he joined the LRA's peace team in January.
The BBC's Sarah Grainger in Nabanga, in southern Sudan, says those agreements have now been called into question as it seems there has been little communication between the various sections of the LRA.
The signing ceremony was supposed to go ahead last week but was postponed at Mr Kony's request, when he told the southern Sudanese mediators he was too sick to attend.
The signing was rescheduled to take place on Thursday at a camp in the jungle town of Ri-Kwangba in southern Sudan.
Ugandan President Yoweri Museveni was then due to sign separately on Monday in southern Sudan's capital, Juba.
The two sides have been in peace talks there since 2006.
Our correspondent says mediators and the United Nations transformed the bush close to the venue of the planned signing ceremony into an elaborate camp, complete with electricity and a hot buffet.
They flew in dozens of journalists to report on the event and had already taken several members of Mr Kony's family to the meeting point.
But there had been fears Mr Kony might not show up, because of the threat of war crimes charges.
If the deal is signed by the rebels and Mr Museveni, the rebels will then have 30 days to gather in southern Sudan before disarmament and demobilisation are scheduled to begin.