Hi @danielleraffoul,
Thank you for your question and welcome to the Bunnings Workshop community, it is brilliant to have you with us.
Diagnosing the issue is obviously the first place to start when working towards a solution. You might want to have a read of this article from @Noelle, one of our resident garden gurus, How to diagnose a sick plant.
Some of the potential causes included environmental stress from over or under watering and extreme temperatures, pests and diseases such as aphids, mites or powdery mildew or nutritional deficiencies, particularly lack of nitrogen. A combination of these might be the cause of the leaf curling.
I'd suggest that you may be watering them too much. If the soil is still damp from the previous soaking, it is not necessary to water them further. You may find that the plants that are thriving are sitting in better draining soil than those where the leaf curl is happening. Too much water can cause root rot which will negatively affect the health of the plant overall and potentially cause the plant to die. Depending on temperatures and drainage, watering for 30 minutes once or twice a week should be sufficient. Check the soil before watering, if it is still wet, you can wait another day.
To improve drainage, I suggest adding organic matter to the soil such as Osmocote 25L Scotts Compost Premium Soil Improver, this will add beneficial microbes and attract earthworms which will help to improve the drainage.
Let me tag @Adam_W and @mich1972 to see if they have any thoughts.
Let me know if you have any other questions.
Jacob
