THE ON-AGAIN, off-again "Nick & Nora" is off again. The $6-million musical, which was to have played the Mechanic during the season that just ended, then was re-scheduled to open the 1991-92 season, won't be playing here next season, either.
According to co-producer Elizabeth McCann, who was speaking for the four other producers, all five decided to open the show in New York rather than do so in Baltimore ''after reviewing the economics and the consistency of the performance schedule.
''We had a very strong advance in Baltimore, and the Mechanic Theatre has been a great supporter of the production,'' said McCann, ''but the take-in and take-out costs alone would be approximately $525,000.
''We feel the work that has to be done on the production can be done during the preview period here in New York,'' she said.
According to Hope Quackenbush, managing director of the Mechanic, the cancellation of the musical, which was to have opened the coming season, ''is a bitter disappointment to us. We held out for two years, hoping the musical would come here.''
Quackenbush added that she has been receiving calls from other producers who have heard about the "Nick & Nora" cancellation and are offering shows in its place. ''I can't say what they are, but two would be tryouts and two more would be shows packaged for the road,'' she said.
''If I can get what I'm after, we will have a show that will please the Mechanic audiences. We will have a good, strong season.''
Quackenbush said that the prime reason for the cancellation of "Nick & Nora" may be the sets. ''It would have taken them five weeks to set them up. 'Grind,' which was a mammoth show, took only two weeks. With sets that complicated, it would take 10 days to move the scenery from here to New York, and that would mean that the actors would be waiting around that long.''
"Nick & Nora" is based on the Dashiell Hammett characters in the ''Thin Man'' stories which were done on the screen by Myrna Loy and William Powell. The music is by Charles Strause (''Annie'') with lyrics by Richard Maltby Jr. (''Miss Saigon'').
Joanna Gleason, Barry Bostwick and Christine Baranski will star in "Nick & Nora" when the musical begins previews on Tuesday, Oct. 8, at the Marquis Theater on 45th Street and Broadway. The show was to have played the Mechanic from Sept. 24 to Oct. 20.