That's because the rest -- about 114 -- have either died, been married off, or become radicalized and don't want to leave their Boko Haram kidnappers, the sources said.
Negotiations between Boko Haram and the Nigerian government over the captives are expected to resume Monday, four days after the militant group handed over 21 former Chibok schoolgirls to authorities in northeastern Nigeria.
Boko Haram kidnapped 276 girls and women, ages 16 to 18, in the middle of the night at a boarding school in Chibok, Nigeria, in April 2014, drawing global outrage.
As many as 57 girls escaped almost immediately in 2014, and one was found this spring
If the sources are correct about the number of dead or otherwise unavailable, that would mean more than 40% of those who were kidnapped in 2014 stand no chance of being brought home alive or no obvious immediate chance of being retrieved through negotiation.
Sources: Boko Haram was paid to release 21
While the Nigerian government has said Boko Haram released the 21 as a result of negotiations brokered by the Swiss government and the International Committee of the Red Cross, it hasn't said what the militant group may have received in return.
Two sources close to the negotiations told CNN that Boko Haram received money as part of the deal. The sources did not disclose the amount.
The Nigerian government has not publicly and specifically addressed the issue of money. Nigerian Information Minister Alhaji Lai Mohammed told reporters Thursday that the girls' freedom "was not a swap."
"It is a release, the product of painstaking negotiations and trust on both sides," Mohammed said.
There was conflicting information about whether captured Boko Haram fighters were released as part of the deal.
The Nigerian government has said there was no prisoner exchange. Two sources close to the negotiations also said no Boko Haram prisoners were released.
However, one source close to the talks said "a number of Boko Haram commanders" were freed as part of the deal to release the 21 former Chibok schoolgirls.
Boko Haram's uprising
Chibok is in northeastern Borno state, where Boko Haram is strongest. The militants were believed to have taken their Chibok captives to the dense Sambisa forest, a reputed Boko Haram stronghold.
The terrorist group, which pledged allegiance to ISIS in March 2015, aims to impose a stricter enforcement of Sharia law across Africa's most populous nation, which has a Christian-majority south and a Muslim-majority north.
Since the group launched an uprising in the northeast in 2009, thousands of people have been killed in the conflict and about 2 million people have been displaced, according to the UN's Office for the Coordination of Humanitarian Affairs. Boko Haram also is blamed for attacks in neighboring countries.
Thursday's release of the 21 came amid an apparent split in the terror group, with ISIS introducing an alleged Boko Haram leader in the summer
even as Boko Haram's longtime chief maintained that he still was in charge.
The 2014 kidnapping prompted global figures such as Pakistani education activist Malala Yousafzai
and US first lady Michelle Obama to support a #BringBackOurGirls campaign on social media.
'I just hugged them'
After the 21 were released in northeastern Nigeria on Thursday, they were taken to the capital, Abuja.
Tsambido Hosea, chairman of the Chibok community in Abuja, met with the girls and helped the government verify their names.
"When I saw them, I just hugged them," Hosea said. "I didn't ask them anything about what they had been through. They told me their names, their local government area (and) their grandfathers' names.
"They were in their senses and some of them recognized me from Chibok. They were very happy to be free. We are now waiting for their parents to arrive."
The former students' release came amid pressure from the girls' families on the Nigerian government to secure their freedom from whomever still held them.
The pressure increased as Boko Haram, or groups claiming to represent it, issued videos over a period of months showing some of the kidnapped students, apparently to prove to the government that some were alive and to make demands, such as the release of captured Boko Haram fighters.
This includes a recording, believed to have been recorded in December, that CNN exclusively obtained in April
showing 15 of the girls.
Another video, addressed to the girls' families in August
, shows 50 girls wearing headscarves. A father of one of the captives told CNN that his daughter and other Chibok students were in the video.
In that recording, a militant says 40 girls were married off. The militant claims some girls were killed in a Nigerian airstrike.
A Boko Haram insider told CNN in August that despite the leadership split, longtime leader Abubakar Shekau still controlled the Chibok girls
and was thought to be hiding in the forest.
According to the Global Terrorism Index released last year by the Institute for Economics and Peace, a think tank, Boko Haram was the world's deadliest terrorist group in 2014. That was before Boko Haram pledged allegiance to ISIS, the terrorist group that controls parts of Syria and Iraq.
That year, Boko Haram was responsible for 6,644 deaths, compared with ISIS, which was responsible for 6,073, according to the GTI.