I would guess that your plants need more water. Try watering well, to completion, so water comes out the hole in the bottom of the pot. Then wait until the soil is dry at depth to water again. If the soil never gets properly wet, your plants will have less of an opportunity to rehydrate before it goes dry again.
Did you recently move the plants outside? Did you give them a gradual adjustment period in bright shade before you put them in the sun?
