Do you explore why adopters want to return pet and offer help or suggestions on how to try to remedy situation. Assuming adopter wanted pet, they might (should) be open to suggestions. Hopefully this will result in a permanent solution, or at the least give you more time to try to find a foster situation. Also a good reason to try to inform adopter of possible issues before adoption (kittens are active, cats need time to adjust an quiet time/space for a while) , cat may be known to like or not to like other cats, dogs. Good luck.
Our community has a large number of seniors (people) and we are frequently getting surrenders or returns from people or their families as the seniors can no longer care for animals. We have established a Senior for Seniors program where we place senior cats in forever foster homes with responsible senior people who want a cat but cannot afford adoption fee or vet bills. This is an expense for the rescue, but we do ask for contribution from surenderor, do fundraisers and apply for grants to help and it is a wonderful solution for pet and forster.