With time running out to reach a ceasefire ahead of crucial November 8 elections in the former junta-run nation, negotiators said they were unable to seal an agreement.
"We could not conclude a deal today because both sides were unable to negotiate the issue," said Pu Zing Cung, of the ethnic group delegation.
He added that ethnic organisations want 17 groups included in the agreement, but the government has balked at including several of them.
Earlier, lead government negotiator Aung Min conceded that inclusiveness was "very important for the peace process".
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But observers say the nation's powerful military is vigorously opposed to including some of the smaller insurgent groups which it is still sporadically fighting on the frontlines, including in northern Kachin and Shan states.
The issue was the very last sticking point in months of talks and it is not yet clear whether another round of talks can be held before the polls.
But the quasi-civilian government, which took power four years ago, has pressed on with the peace negotiations that it has placed at the heart of its reforms.
It sees a nationwide ceasefire as opening the way to more complex political dialogue and questions of federalism in a country where the army has for decades hung its legitimacy on enforcing its own concept of unity.
However, elections in November may reset the negotiations if a new government comes into power, with Aung San Suu Kyi's opposition expected to win large numbers of seats in the polls.
But the government is eager to seal a full nationwide ceasefire before elections which are seen as a key test of reforms after decades of military rule.