Powerlessness corrupts, and we get discouraged when we seem to have no control over what happens in our tanks. Sometimes we have to focus on the tank itself.
You can be the best fishkeeper in the world, but if the animals arrive compromised - if they had a rough time before you got them - there's nothing you can do. Certainly, good care can bring them around if they healthy enough to respond, and they can live long lives, but if what they've faced has been too much, then it will play out.
You are in the US, and
@Abaddon is in the 6th circle of hell, so location doesn't help. It may matter though. This issue comes up a lot,so I may be repeating myself (sorry) but large areas of rural North America have water problems with nitrates from farming in the water. How is your tap?
Large areas of North America are as hot as
@Abaddon 's environment is said to be in the old tales, so what temperatures are these fish enduring? This is a season where fish die in spite of our best efforts. Your tank may be considerably warmer than water in Mexico where your platys or apple snails originated.
What do you think of your source? Is it a good local pet shop - clean, well run and well informed or is it a quick turnover chain store, designed for shareholders and not customers? If you bought them online in the fast couple of months, they may have encountered killing heat.
You aren't describing any symptoms that make diagnosis easy, because they probably aren't there. What's happening is inside a shell, after all. Older aquarists like me will think you've hit a bad patch with unexplained fatalities, because it happens to all of us (and to our fish). Sometimes it's our fault, but sometimes it's the environment we can't control. The dead of summer with new fish and snails is tough. Once the tank ages in and is established, it gets easier.