Are you getting any error messages?
Can you export the file in two halves or in thirds?
No error message, just nothing happening. Haven't tried the halves or thirds but will do so this afternoon. checking fonts for embedding permission has resulted in some changes in the copy generated from the .idml,but it hasn't changed the response. I'll be back with report on the halves or thirds effort.
Thanks, Peter. Exporting one page at a time narrowed the search down to a problem page, which turned out to have a problem with a graphic used in every issue. Replacing and relinking the graphic got past that hump. This was on the new file copy which had gone through all the other changes previously described, so I don't know if they had any effect. Need to get on with other back issues rather than experiment with the original file.