Sources connected with the former couple tell TMZ ... Mariah has been trying to get Nick to sign off for nearly a year, but he just won't do it and won't say why. The couple separated in August 2014 and Nick filed in December of that year.
As we reported, in December 2014 Nick and Mariah struck a property settlement agreement and they agreed to joint custody.
We're told all of the issues have long been settled, yet Nick repeatedly has declined to sign the papers that would make the divorce official.
Our sources say Nick -- even though he wanted out of the marriage -- still loves Mariah and doesn't want to officially let her go. He also doesn't want Mariah to marry her fiance, James Packer.
Our sources say Mariah is frustrated and not flattered at all. She wants out ... and now.
We reached out to Nick ... so far no comment.