Are there tiny white pin-dots on the top surface if the leaves? ...especially along the center leaf vein and radiating outward?
If so I suspect sucking insects on the underside of the leaves. If you can't actually see any, try putting a white paper underneath and flicking/tapping the leaves from above, then placing the paper on a solid flat surface and strongly rubbing a finger across the paper if you see fine particles on it.
Another way to check is to make a glass or clear plastic cup of sudsy warm water, then vigorously swishing some of the leaves in it -- on or clipped off the plants. Then remove the leaves from the water. (IF YOU LEAVE THE LEAVES ON THE PLANTS, BE SURE TO IMMEDIATELY RINSE OFF -- within 5 minutes) Look at the water in the glass from the side with bright light behind it and see if you see anything floating in it.
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