The only time I've had that in the past was due to heat issues. Just because the temps look good when you check them doesn't mean they've been good all day. Get a MIN/MAX thermometer that'll keep track of the highs and lows so you can see how hot it really gets at certain times of the day.
Also might want to check to see if the sipper tubes on your water system are clogged. That'll kill em pretty fast too.
Don't always assume the worst, start by troubleshooting the simple/obvious things first.
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