Yeah, some cats seem to do well with others, some don't. My first cat, Apollo, desperately needed a friend, and it was fairly obvious because of how much he acted out, wanted to play, begged for attention, etc. As soon as we got Athena it was like his world was made 10x better. Athena could probably go either way and is good at entertaining herself and sometimes wants to be left alone, but also seems to enjoy having someone to play with now and then.
But other cats really seem much happier being the only cat.
Could be Casie just prefers to be without the company of other cats. As long as she seems happy/not lonely or bored, then I wouldn't worry that you did anything wrong by not introducing another cat. She may have reacted badly to another cat if she's not fond of playing with them.