Q.why is English Spinach curling up
I have planted English spinach in planter boxes in my back veranda. They get morning sun and have produced fantastic leaves for the first couple cuttings. Now some of the plants are starting to grow inward to the stem, looking like shell pasta. Half the plants are now growing this way. I also feed the plants once a week with NPK 5 in one liquid fertilizer.
Leaf curl is likely due to Aphids. You can use a blast of water to wash the leaves.
Insecticidal soap can be used to control Aphids.
Here are a few links for you.