Gluing a headliner that has disbonded will not work. OK, it might work for a very short while (a couple weeks), but not over the long haul. Regluing an area that came unglued does nothing to answer the original problem: why did it debond to begin with? The only long-term solution is to pull the headliner out, scrb down the cardboard to remove all the old adhesive, put new adhesive on, and install a new headliner cloth. The old headliner cloth will have old, failed adhesive on it and won’t work.