How do you fill in low spots under an above ground pool?

This is not the answer you're hoping for but it IS the answer. You're going to have to remove the liner. Fill in the depressions/low spots with sand & a little of dry portland cement. Dampen the mixture so it will hold together and put it where the depression(s) are. After you do this, there is a product called Perfect Pool Bottom made especially for above ground pools. I believe it also comes with the "coving" you should have against the pool wall at the bottom so your liner will not be forced under the aluminum walls by the water pressure. It'll be a little work, but it is fixed.(period) Good Luck