I love plakats.
Hopefully you have better luck with your new additions. I haven't had any problems with my 2 and one came from petco one from LFS, but I have had problems with other fish from certain stores, so I understand issues with certain stock.
You could try running salt in their tanks if you don't already. I personally am not an advocate for salt use in general, but many people swear that bettas in particular do better with it.
What kind of live foods were you feeding? Some in particular can harbor nasty bacteria or act as an intermediate host for certain parasites.