The item appears to be inside a group; try to break inside the group and then do the section hatch. If that does not work , try exploding the group and use the levels and layer tool to organise your model.
In rare cases, old geometry that has been altered will not hatch; if this is the case click on the slab, open the slab tool, and click submit. This will redraw the slab, and the section hatching will work. I hope that helps…