My vet also recommended baby wipes. Regular pet wipes don't need FDA approval. However, the baby wipes do. I've also used the small soft baby wash cloths (with warm water) that I've purchased from the Dollar Tree (4 for $1).
Our new rescue cat Orson was having this problem, too - she is 1 years old and I noticed she was not cleaning her rear very well. I put a dab of very soft butter on a baby wipe, then swabbed that very gently on her bottom. She immediatly started cleaning herself, the other older female cat Emily gave a few motherly licks, too (something which commonly helps little kittens to learn how to clean themselves).
5 days on and it seems to have worked: little Orson is now cleaning herself more thoroughly
I was so happy to find this thread. In over 50 years of owning a cat, I have never had one that didn't clean his/her butt. Until now. I adopted a Siamese Red Point male, age about 2, four months ago. SO cute and sweet. But he gets dark crusty discharges from his eyes that I need to clean every second day. And dark crusty clingers to his butt. So undignified! Thank goodness he has such a great personality that I can easily clean his butt with a wet paper towel. But I don't much enjoy doing it. After reading this forum I put some butter on his butt three days ago. He licked his behind (something I've never seen him do before) for hours that day. He's still clean so I am crossing my fingers!