Translation:Their relationship was good, but not anymore.
I was also confused, but I think this may be idiomatic! https://context.reverso.net/translation/french-english/mais+plus+maintenant
You can tell immediately that this is negative by the way the robo-voice vocalizes this. If it were positive, as in "more so", you would hear the s in plus. Here you don't hear it.
Also, you don't have ne because this doesn't have a verb, and ne goes in front of the verb. It is an abbreviated version of "mais maintenant elles n'étaient plus"