I've only had a tank going for 3 weeks but I have a betta, and a mix of mollies and platies. I can NOT keep a platy alive. Mine all just up and die. No clamped fins, no obvious wounds, parameters great, no aggro from anyone, not even the betta (mine never bothers anyone, the most I've seen him do is flare his gills or give someone the you-best-be-moving-on look). But I've noticed all the platies I've gotten, they seem to be so prone to stress, they hide more, water changes freak them out more, they linger before going to eat when fed and hardly get any, the don't bounce back from labor as good as the mollies, they are just more skiddish in my limited experience. I've had three now. I am trying one more because asthetically I prefer their appearance to mollies and by far their more sedate behavior is much more relaxing to watch. But for now, it seems to me that they are just more prone to stress-issues than mollies.
Have you got plenty of hidey spots? My betta is hardly aggressive at all. Like I said, he will "defend" his territory, which is the back right corner where the driftwood reaches up top. I put several plants there for him, and a leaf hammock. We looked underneath before adding him and based on the stillness of the water, we figured that would be his spot and were right.
Maybe take him out, do what I did, check the surface area, then rearrange the tank decor and plants to sort of "shepherd" him to a spot that stays stillest (assuming you have a filter). My layout is very open yet also compartmentalized. It's almost like a house with an open-concept design, and I think that helps everyone feel like they have a spot, yet also keeping it open where we can enjoy watching them.
My bettas: D'Argo + the girls Jullanar, Lalla, Razz, Rayleigh, Aviendha, Arya, & Lovelylocks
plus the other guys Toni, Eli, and Clinton (platy, guppy, and molly) a kuhli- Burt, 2 yoyos- Duncan and Ma, and otos, Laurel, Hardy, Larry, Curly, and Moe, plus the kitty, Clara the Impossible Girl.
SIP: Turk (teal VT), Azul (royal blue VT), Jerrica Benton (pink cambo CT), Lady Amalthea (blue cambo CT), and Jadzia Dax (blue VT)