There are several possible reasons why your plants’ leaves are curling. Considering the hot, dry weather we’ve been having it is very possible that the leaves are expiring water faster than they're taking it in from the soil.

The other possible cause could be aphids or spider mites. Both are small and may be missed when looking at the plants. The aphids usually live on the underside of leaves. The good news is that they have predators which may have found them, so even though they’ve already done the damage, most may have been eaten. 

The problem is that if you have spider mites, aphids, or fungus, you can spray to prevent further damage, but the chemicals do not cure the damage that has already been done. However, curled leaves, while unsightly, can still photosynthesize and produce food for the plant to continue growing.

You may find the following articles helpful. Foxglove aphids: Gear up to deal with this cooler temperature greenhouse pest - MSU Extension and Leaves on foxglove beardstongue - Ask Extension.