well, i am glad you are taking her in.
it still made sense to anticipate the med - the earlier you catch the infection usually the better. perhaps she is just taking more time to respond. the vet may give her a shot to build up her antibiotic level sometimes they do that usually the first dose but...
if thinks she is not responding at all i wonder if they can do a culture to check for resistance. i don't know if they do that with cats as they do with people.

keep us posted.
glad the other two are shaping up.
sending you more kitty-healing wishes...
~ waves ~