I wonder if the "slow release" nitrogen is killing off your plants. I would figure it has to be one of three things.

1. Wrong soil
2. Overwater
3. Too much heat

I pick the first one based on the info you've given here.....for one thing you say it's happening to your plants over and over so it's something you are continually doing. I think that lets out the heat (JUST GUESSING) and it just plain looks like overnute burn to me.

The only way I know how to fix something like that (without being able to pinpoint the exact problem) is to change things one by one and see how your plants react. I'd start with the easiest thing first.....change watering schedule perhaps? Or move from the sun and heat into the shade? See what helps and what doesnt. (this is one of the things about soil....the time it takes to figure this stuff out...)

My gut feeling is that is your soil (the words slow release make me shudder)

Good luck.