It seems that those plants have not had enough time to establish an extensive root system and are suffering from a lack of water in the soil. Once such damage occurs symptoms can progress for a while even if you have watered or if it has rained. Try to keep the soil moist but not soggy wet as soggy soil is also very damaging.
If you put fertilizer in the planting hole the salts in the product can burn roots causing symtoms similar to what you described. The only solution is to drench the soil thoroughly to wash out the salts, but don't keep it wet for long periods of time.
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