Female Betta not doing so hot

[Cclune39]

Hi there! I recently started a female sorority tank, I originally had 3, noticed one (Pinky) wasn’t doing so well so I went back to the store and the lady there was really nice. She’s worked there for years and has always had really good advice regarding my fish; she suggested I add one more fish to the mix so I did just that and they’ve all been doing very well together, but Pinky is still having issues. My biggest stumper here is all of the other bettas are great! They’re doing amazing but Pinky just isn’t. She’s lying on the bottom of the tank or staying at the top in one place for extended periods of time, all of her fins are straight out, and she’s not eating with the others. The temperature is 78, the tank is a 20 gallon and was cycling for a while before I put these girls in so I have no idea what to do or what I’m even looking for.