What is your secret to backing a quilt?

I have been helping my mother to back the quilt she has been making. We layered the bottom fabric out flat, laid the wool thing over that and then the quilt top on top of that. Made it as flat and taut as we could. transferred it to the table. checked the flatness again, and then started pinning it. 3 hours pf [inning later we flip it over and find uneven loose fabric. So Have to unpin the area effected. 1/2 the quilt just to flatten the fabric again and then re-pin it. Now we found loose fabric elsewhere. 😱 We have never had so much trouble with a quilt before.