I would venture a guess that it could be a water quality problem. (It could also be a genetic problem (very unlikely) or a disease problem.) I'd suggest doing things differently in whatever ways you can think of, and you'll hopefully hit on success. Some specific ideas are:
1. Try doing much more water changes, like half every day. And larger water volume too. They will still find the daphnia.
2. Try using bottled spring water for some, in place of aged tap water.
3. Check with your local waterworks to find out if they have switched from chlorine to chloramine. Many have, and if so, aging for 2-4 days is not adequate, you have to use Amquel or similar product.