Spray water with a plant sprayer on both the chimney breast and the gypsum and you'll see your pl400 adhere brilliantly. Now, tons of know-it-alls will claim that you'll get mold, water damage, and have to contest invoices for the rest of your life, but don't worry, the little you apply with the plant sprayer is no worse than a sneeze.
If the plaster is very bad, coming off and just crumbling, I would have glued the board with gypsum mortar and fixed it with spike plugs. The mortar evens out irregularities so they don't become "hollow" and the spike plugs hold the board in place.