How do I fix A sagging headliner in a 89 buick riviera?

I removed the headliner pulled the fabric off and re-glued with 3M adhesive. Two weeks later the headliner is sagging again. Anybody know where I can find a replacement?How do I fix A sagging headliner in a 89 buick riviera?Did you sand the old foam off the headliner board first? If you do not the adhesive has nothing to adhere to.