Could it be that you are using Automatic Press Sheet Size or a sheet size not big enough to fit the bleed? In that case, try increasing the Margins in the Gutters tab of the imposition.
After performing the imposition there are two options that I am aware of for creating a bleed.
You can use the Optimize node to create a Bleed using scaling -- see screensnap.
Another option is to use the workflow defined here https://forum.support.xerox.com/t5/FreeFlow-Core/Adding-bleed-to-documents-by-mirroring-content/m-p/... which describes creating a mirrored bleed. There is a zip file with the workflow on the page "AddBleedbyMirroring".
Making a workflow with two impositions (14x1, followed by 1x4) that makes a total of 56 artboards turned into a single page for printing. The outer edge of those 56 artboards as a whole needs a bleed. I was using Trueflow before, but we've switched programs. Am I missing something right under my nose?
Thanks for any help
(Also I can't figure out how to add images for helpful visual aides; sorry)