Exterior Slate Help.

My parents are going to be moving out of thier house in about six weeks to
relocate south.  They have slate pieces on the top of thier porch and steps.
About three of these pieces (about 12" x 18") will note stayed adhered to
the surface below which is concrete.  They need to fix these pieces before
the home inspector will give the home his blessing in order to close.  Any
thoughts?  We've attempted to fix these three pieces several times over the
years, but nothing seems to work.  Any advice is GREATLY appreciated!!
TS